For anyone that runs Joplin Server on RasPiOS 64-bit, a Bullseye version has been published today. It's not linked to from the main site so I guess it's still classed as a beta. The RasPiOS 32-bit Bullseye version is available from the usual Raspberry Pi software download page.
Apologies The 64-bit link above is for the desktop version. The "lite" x64 version is here .