Offer Rescinded

I think in this case you would have the right to a conversation, ask HR why the CEO welcomes you to the team and then the offer isn't there any longer. Not only makes behavior like this make the firm look bad, it also is a strong warning signal to other applicants not to go here.

Legally, I don't think you have anything on your side, they never extended an offer or signed contract.

The only thing I could imagine is that your background came back with entries (criminal, bad credit, education unconfirmed, etc)

Did you say something like “I’m excited to work at megafund” instead of “I’m excited to work at [platform name]”?
 

This whole thing seems bizarre

This is very strange. As others have said I would reach out to HR and ask what happened. Either something strange happened or you aren’t reading the situation well (If CEO hated you they can override the decision). 

What level role is this? Executive? What CEO of a mega fund is reaching out to new hires on a call?

I can offer a few "maybes".. frankly very much guesses in your situation, but I know these can and do occur...

 - Budget/position got pulled - after the "CEO of platform" sent over the request to get you an offer from HR, something triggered a review or something caused the position or budget for position to get pulled by higher-ups (or more like was never actually fully approved). It's an odd fuck-up, but it happens. For it to happen so "overnight" seems a bit odd tbh, this more logical if you had been ghosted for awhile after the call and got the HR note/call like a few weeks later, but I guess overnight could happen. 

- Someone in HR or elsewhere canned you after your name got pushed up for an offer. Why would this occur.... most common reason where I have heard of this occurring is something "unflattering" comes up in a google/social media search. I've legit heard of this happening (from people who were on hiring committees), they may never tell you this for liability reasons, but it is super common. Clearly, you should know if this is a possibility or not, but it would fit the random/fast timing. 

- I guess the CEO could have been lying by welcoming you to the team, but that is really illogical and hyper-stupid on their part. Smart thing would have been just to end it by saying "we will be in touch", unless just a supreme dick this approach is crazy. Frankly the bad firm reputation (like if you say who in this forum...) is not worth fucking with people like this. Possible? Yes. Likely? NO. However, a subset potential is that someone else you interviewed or interacted with had meant (or thought they had) veto'd/killed your hiring, they responded forcefully by news an offer was forthcoming. Again, I don't see likely as this person would be literally contravening the CEO, which takes some balls and/or one hella position of power (including an admission of fucking up for letting it get so far). 

Other than those, I have no idea, but this is blind speculation of course. What would I do.... I'd call and email the CEO directly and in very polite, yet assertive means approach like "Hello, I received this note from HR, was there a misunderstanding, can you explain or do I need to provide additional information?". I'll admit, a "mistake" like this by HR is virtually impossible unless they just suck and some other candidate has nearly an identical name or something, but this is the "face saving" way of approaching the CEO in the hopes of some reconsideration. 

I’ll add one more speculative thing because it’s fun to speculate and no one said it yet. It’s horrible that they gave you a verbal and pulled it. For all you know someone’s son/daughter/nephew/niece/top clients kid needed a job and some string got pulled, taking the job from you and giving it to them. 

You should 100% call HR and inquire. Not because it's going to make the outcome less shitty but you just want to makes sure there isn't a loose end out there that will continue screwing you. They may not tell you but if not I'd say something like "Look, I know this is a liability thing for you guys, but I'm just looking for a frank answer so I do not have the same issue in the future" 

If I had to speculate perhaps someone at the company called a former coworker or mutual connection and got negative feedback on you. I have heard of that happening although typically it's during the reference check or before someone gets a verbal offer.  

HR isn't going to tell you anything, per company policy. The most common reason I have seen in this situation is the discovery of a tweet  or similar  social media post that was not on brand for the company. 

Tough luck. Supper shitty of them. Sorry. It’s shitty when both parties do it. It really goes both ways. We had two candidates (not just one off odd one but two coincidently) back out after  us extending them an offer, them accepting it, and we even got to the point where we were asking them their address so that we can send them their laptops. both of them decided to stay at their current place, and we are assuming they got a pay bump. 

That sucks, man. Hopefully you can get something else equally as good. Idk why it happened, maybe they gave the offer to someone’s nephew/brother/cousin.

Nothing you can do...move on and find an even better role and know that at some point in your career down the road you’ll have the opportunity to fuck them over in the same way they did to you, just wait for that shot.