or somehow the database folder disappeared.
As I said, on a clean shutdown, the database folder is deleted by newer versions. 0.19 is new enough.
Yes, I have observed this behavior, and now it makes sense. It explains why there is no database folder. The wallets work when I loaded them in Knots 29, with labels and all. The HD logo has a crossing line on it indicating it's not an HD. So the experiment may not have worked because Tails did not properly shut down the wallet and I also did not copy the database folder. But the actual wallet did work thankfully, now I need to plan on a watch-only setup.
An observation: When I loaded the wallet.dat file on Knots 29, the sha256 checksum changed. What changes are made to the original file?