OK, so I managed to get it to work. I had a stray file in the SQ5 directory that ScummVM somehow picked up even though it shouldn't. Once I got past that, I was able to look at the bug - and yes, it is a bug in ScummVM. It happens in a part of ScummVM that tries to synchronize the in-game settings with those of the main ScummVM UI. For various reasons (mostly related to SCI32 games), this code has a hard-coded list of games that support text and subtitles at the same time. Unsurprisingly, SQ5 is not among them!
I'll contact the right people next, so we can get this sorted (I do have commit access, but I haven't used it in years).