Hi Matt,
So I tried with another PC and I reproduced the same issue. I tried with several DVD (my japanese import, US or European ones) and get issue with all.
I noticed something: if the DVD has internal subtitles (so the encapsulated Idx+Sub image format), at the beginning I have obviously no issue to navigate and get them. But if one is active and I load a srt (or other text based subtitles) through the "Browse" function, the srt file is indicated as active in the Subtitles sub-menu, but it is still the images based subtitles from the DVD that is really keeping to be displayed. I need, still on the subtitles sub-menu, to click on "off" and then on the srt file added entry to really make the switch. But then, the DVD encapsulated subs are not working anymore if I try to switch to them. I feel like a subtitle renderer (one working with image based subs and the other working with text based subs) switch is forced by my action. Furthermore, if I click a second time "Off" on the subtitles sub-menu and then one of the DVD subs track, no subtitle is display anymore, like no switch back to the initial subtitles (image based) renderer is possible.
I hope this make a little sense to you. I can try to provide you sample but I got this issue with all my DVD so I'm not sure this will really help you.