Hello. This has been a problem happening since 5.9.3 and it still persists in 5.9.4. It did not occur on previous versions of the app.
Basically, while downloading files from the app itself, the video will have inaccessible data. I say "inaccessible" because if you scrub within the affected region, it shows the thumbnail but play anything within that region, it refuses to play and skips to a part of the video that it can play. Sometimes, you can play the affected region, but severe artifacts will show. The missing data varies depending on the length of the video. For movies, you lose several minutes. For TV episodes, you lose a minute or so. However, this is anecdotal and it could vary.
Downloading apps is purely when it is initiated in the app. I have faced this issue in these two places: One, directly by accessing the Plex server's internal hard drive location and pressing the Download icon beside the file. Two, from a user-set favourite Plex library and pressing Download when selecting a specific episode or movie.
The problem does not occur in File Sharing in iTunes, as it uses a different transfer method. I would assume the corruption does not occur if you use Wi-Fi Sync in iTunes.
I found a very severe one, using the movie Frozen as an example. (Of course, it happens in all files.) https://drive.google.com/file/d/1KIwuDhMAxb7JJ...
Usually the corruption is not that severe. I've faced just several minutes in TV shows, more in longer length files like movies.