After updating from 0.6.21-ls256 to 0.6.24-ls299 fb2 format is missing when accessing calibre-web using opds. FB2 is still available for manual download in calibre-web interface.

Calibre-web 0.6.24 (Docker, linuxserver 0.6.24-ls299)
Moon+ Reader 9.7 from GooglePlay

I suspect commit 09c0794 to be the cause (application/fb2+zip => text/xml).