SOLVED: It is not possible to fast-forward or rewind a series; it always jumps to the beginni

[Jelly-User]

I use Handbrake, convert the footage to h.265, take the audio from the original file and use MKVToolNix to create a new .mkv file, just like I always do.
There is nothing special about the files, I have already used the same settings and procedures for many series.
On the PC, I can go back and forth using the Jellyfin app, and then start again, but on the TV, using NVidia Shield Pro with the Android app 0.17.7, it doesn't work, at least not with this series. On the PC, I had virtually placed the cursor at the point where I wanted to continue watching, on the TV I had gone to “continue watching” and the episode had then started again from the beginning, even if I selected a different chapter.
My approach to creating.
Use makeMKV to play BluRay on PC, use Handbrake to convert the episodes, use MKVToolNix to mix picture and (original) sound together. Use Filebot to name the episodes. Then use MetaX to tag the episodes, similar to mp3 files. Done.