Thank you @td47 for your suggestion, and sorry for the late reply.
I didn't have a previous TV, this is my first one. The firmware was updated right after unboxing the TV, so I didn't have the chance to try the movies with the previous version.
I've tried only MKV files with different codecs (h264 and h265) and resolutions (720p, 1080p, 4K), but none of them worked.
I will try with some MP4's and let you know.
Just to be sure: you're telling me that resume / fast forward should work on any file?