I'm on the latest version (14.75). If I try to watch a long video (like 6 hours), it doesn't start from the beginning and only its last 2 hours are playable.
This happened last week as well, but I didn't realize that it was happening since it started from roughly the same spot where I stopped watching on my PC and I thought it was intentional from the app to save ram or something.
Edit: It fixed itself overnight? I didn't change anything nor update the app, I have no idea.
No one is going to be able to help you here. Submit a bug... https://github.com/yuliskov/SmartTubeNext/issues