I see this regularly. Listen to a video, then stop play, and go to sentence mode. Switch to another app, to check a word meaning for example. Return to LingQ, switch to play mode, and the video is black and there is no sound. Generally switching back to sentence mode, then back to play mode, and moving to the previous line prompts it to restart the video, after a longish wait. Note that I switch to sentence mode as a workaround for a bug that has been present for years.
There’s countless other bugs. With some videos the current place is lost, and the following day it thinks I’m still at the start. This is incredibly annoying as I lose my progress. When I edit a sentence, it now shoots back to the start. This long standing bug had been fixed, it’s back. A more recent one, click on a word, the meaning dialog pops up. It can take ages to populate the meanings list, sometimes it never appears. This is ‘fixed’ by disabling the AI meanings.
Bugs sometimes are fixed, but they usually reappear a week or month later.
iPad 12.9” iOS 26.4.1.