Exactly...well, something like this?
I was writing my response at the same time as you were writing yours, so i didnt see that you already added extra logging...that was the debug logging that I added. My assumption is that the current position skips backwards in the TsReader for some odd reason. So this could be related to the seeking issues I and other people are seen with TsReader.
It will be interesting to see what the result is, but lets hope i made a mistake in SubtitleRenderer, since it will likely be easier to correct than a seeking bug in TsReader