I have tried changing the engine with both the newer and the old versions of the app cannot make 1.11 stick. The older version works fine with 1.21.1. Let me do a backup and update to try to get a screenshot.
I originally went
Version 8.8.15-fdroid (4865) to Version 8.8.30-fdroid (4895) to Version 8.8.40-fdroid (5098)
Now I went
Version 8.8.15-fdroid (4865) to Version 8.8.40-fdroid (5098)
and the issue persists in the newer versions.
Also the description in f-droid for librera states:
“For better security, the F-Droid version, is based on the latest MuPDF 1.16.1 and does not require access to the Internet.”
But 1.16.1 is not even an option now.