I've been encountering the same issue after using groupy for maybe a week or so (by now I'm one month in). I have a high-res laptop screen with 250% scaling that I frequently connect to and disconnect from low(er)-res external monitors w/o rebooting. As described in the issue, the titles of various non-grouped windows are tiny, as if the texts were shown at high resolution of the laptop on the low-res external monitor (as it occurs with apps w/o proper support for high-res displays), except they are tiny also on the high-res laptop monitor. My guess is that it occurs when connecting and/or disconnecting to the external monitor w/o rebooting, but I haven't systematically tested this.
Today brought a new wrinkle: Instead of being tiny, some titles (the usual suspects) are suddenly huge, as if the texts were shown at the low resolution of the external monitors on the high-res laptop monitor, except they are huge also on the low-res external monitor -- so the exact opposite of the tiny titles issue, but surely caused by the same underlying issue. (The titles exceed the height of the title bar, as does the background color behind the text.)
(Probably the issue is the difference in scaling between the monitors rather than the difference in resolution as worded above, but I'm not an expert in this.)
This is not a deal breaker (groupy is too useful for that), but still a bit annoying and a fix would be greatle appreciated. Thanks for looking into it.