I have a bug report on file about this issue, and received the same answer, "we no longer support stacking stacked files". I reviewed the "What's fresh" release notes for all versions back to 1.9, and there is no mention of this change -- it was done without informing the users, so "its not a bug, its a feature" won't stand up.
Actually, I would like to see both the old and new functionality available as an option. It would be useful to be able to eliminate subframes with streaks and airplanes through them. But, the users who are keeping all subframes are probably doing their own stacking on a PC with Siril, or some other package, and probably aren't using Seestar stacking.
On the other hand, if there is nothing wrong with the Seestar stack, then it saves time to combine several night's work by selecting just a few (stacked) files, rather than potentially hundreds of subframes. That's why I would like to be able to do both.
Please convey this to the development team
