I have the same bad experience with the sources, the solution is very annoying, but my fix is like this:
Try and let Jellyfin identify the metadata, if the poster is without text you should tap the three dots, choose edit images and on the primary image click the magnifier, then you might get some options to choose another poster, when you have the one you want tap the download button below the image.
I have made 2 type of libraries, one the saves metadata to the media files, then i have another library for movies which only has read only permission where i movie the movies to with the metadata. On this library i have disabled all external metadata sources so it use the local metadata and cant overwrite the metadata i have accepted.
This especially started happening after 10.9, i find it really annoying that we as admins cant choose to prefer meta with text or without because its a lot of work.
Server: Intel NUC N5105 - 32 GB RAM HDD: OS: 1 x WD Red SA500 2TB + 2x 8 TB + 2x 4 TB WD Red Plus in 4x LC-35U3-C-HUB OS: Debian 12 + Jellyfin 10.9.11 Docker Network: 2x ASUS GT-AX6000 + RT-AX92U in Aimesh