I tried adding a tag to a bookmark but the "update" timestamp wasn't updated.
Nice catch, the implementation only tracks insertion date. Will put together a fix shortly.
Made a fix for this in: https://git.sr.ht/~mlb/linkhut/commit/80a3433319166a866d2129f8e1cae242ec5b436e
This is now deployed in the flagship instance, feel free to test it out and let me know if it's still not what you expected.