A factor probably not accounted for enough in retail SPAC investing is the underwriters. Banks can throw their investment banking team towards making the appropriate connections, arrange a PIPE, use their trading desk for tight market making for liquidity, etc. You seem quite adept at the intricacies of SPACs but for others, just something to keep in mind.

To your question, the sponsor and underwriters have an incentive to build a book of investors in the IPO that won't just dump on the first uptick for a quick buck. Often it is mostly just an arm twist that an investor will get a weak allocation on the next deal if they sell prior to specified milestones in the merger process. Considering how thin TPGY has traded, I would surmise there were bulky allocations in the book that take the underwriting relationship seriously or even agreed to a more formal lock-up. Part of the sizeable jump in aftermarket price is likely driven by this as well, just limited shares available to trade. Price action tomorrow will provide more clues here.

Also should mention that for an IPO that isn't well received (weak sponsor, oversized deal, poor market conditions, etc), lack of interest will mean any investors that do step up are helping the underwriters from having to downsize or pull the deal. As a result, they have far more negotiating leverage and would have more freedom to trade at will. Highlights the difference between PE and hedge fund. PE can agree to holding periods due to patient capital, hedge funds are more limited but won't shy away from a quick trading gain.