Looks like firmware 1401 breaks the subtitles for files played from USB devices.
Once the first one appears, instead of ending when it should, it stays visible forever or until the next title replaces it.
This happens on my Q90R and I also tried a total factory reset but it didn't fix the problem.
Yep, I have the same problem here (Q70R in this case), though to be more specific, there is a bit more to it than that. At first, the subtitles actually don't appear at all (even though they are enabled in the settings). You have to either close the file then open it again or rewind/fastforward a bit to make them appear. Then, when they finally show up, as you said, each subtitle stays stuck on screen until the next one appears, which then stays stuck on screen until the next one appears, and so on. So, depending on when the next one shows up, you can be stuck with a subtitle on screen for a lot of minutes, which is quite annoying.
I too have tried a factory reset and it didn't fix the problem. Makes me regret I didn't disable automatic updates, especially since chances are they probably won't fix that problem until 6 months from now (based on previous updates problems I have seen mentioned on this forum).
I have the same problem with files streaming from a dlna-server on a 43" Frame from 2019. Unfortunately there is no way to downgrade to the previous firmware, so we will have to wait for the new one to see if it gets fixed.