It has happened to me a few times already that I get the pdf from the airline and Android conveniently offers to open with itinerary. That works until I get a passive message saying that importing failed (I guess I could look up the exact message if you don't know what I mean by that). If then I navigate through the application into importing a pdf file, then it works. Which is great, but a bummer that I had to massage it that way. I'll gladly share any further info or documents but as I said, I've the feeling that it always happens so it doesn't really depend on the document itself or the state in my system.
This is definitely supposed to work. Which Qt version/package is this? We needed several Qt patches in the past to make this work, maybe something got lost again in the recent 5.15.5 update.
I'm using the one from f-droid, so yes 5.15.5.
Hm, can't reproduce with the latest 5.15.5 based f-droid APK here, neither coming from the file manager nor from K9 Mail. The necessary Qt patches are also still included it seems. So this is due to specifics of the Intents send by the other application?