I have no idea why Safari Technology Preview has not picked up your bookmarks. I just searched my system using FindAnyFile and I only have one set of bookmarks for Safari and there are no separate bookmarks for Safari Technology Preview confused

Safari Technology Preview is in a sense a Beta, but a bit more than that. It is used to try out potential browser features to see if they are worth developing and including in a future release of the OS and Safari. The actual Safari beta is only available as part of the OS beta. (Yes, there are links to a Safari beta, but they end up re-directing to Safari Technology Preview). Safari beta issues are reported through the macOS, iOS, or iPadOS Feedback Assistant that is installed as part of the beta OS, while Safari Technology Preview Feedback is a separate system accessed from the STP Help menu.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein