Dear @Lauri @martin_kemner @msanii and All,
As you already seem to know, this dialog was introduced to warn users that they were running a 32-bit app. This was in preparation for macOS Catalina, which is 64 bit only and will not run any 32 bit app.
After researching this a bit I found out that, for any given app, this warning is shown every 30 days. So that probably explains why you are seeing from time to time.
But the question remains, why is it being shown for a 64 bit app?
I can only guess that the answer lies in the two background applications used on MacOS to play movies -- the 64-bit IzzyMoviePlayer64 which uses AVFoundation, and the 32-bit IzzyMoviePlayer32 which uses QuickTime. When Isadora launches, it launches both of these apps and I suppose that the launch of IzzyMoviePlayer32 triggers the warning.
(While researching this I found a nifty app called Go64 that will identify all of your 32 bit apps so you'll know which ones you need to update for Catalina. It listed Isadora as a 64 bit app -- but of course isn't looking inside the "application package" to find IzzyMoviePlayer32.)
In any case, rest assured that Isadora 3 is in fact optimized for your 64-bit version of macOS. We know the 3.0.2 version crashes under Catalina because it tries to launch IzzyMoviePlayer32... but the 3.0.3 version, which does not attempt to launch IzzyMoviePlayer32, seems to be running well on Catalina so far. (We're continuing to test.)
I hope that answers your questions and lays to rest any concerns.
Best Wishes,
Mark