Thanks for this (and for all the trouble you went through to look into it!). My drive headers were still using LUKS1 for some reason, so I booted up a live image, converted the headers to LUKS2, rebooted, and Dolphin didn’t care. All the tabs still show Xx T/GiB Encrypted Drive, even though luksDump shows they all have lovely LUKS2 headers, and lsblk shows the drives show up with the expected names.
I’m glad to hear that you found a fix at some point, so I’ll keep searching. I really appreciate your help!