I grouped several apps inside a folder. After that, I cannot modify the app order inside the folder.
I believe this is a bug (hope I put the topic in the right category; please move it if not)
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Welcome in the forum 
Sadly it’s not a bug. For now Bliss Launcher doesn’t allow to move icons in a folder.
It has been mentionned in the following Issue :
move app icons inside home screen folders
Great to see it was already reported, thanks.
I looked for it in this forum but couldn’t find any post 
Strangely, the launcher also does not allow you to move apps OUT of a folder… see https://gitlab.e.foundation/e/backlog/-/issues/796#note_50106 If you vote (‘thumbs up’) for the topic on Gitlab it will get more attention!
I encountered this frustrating half-bug too. Thanks to your link, at least I was able to use the workaround of first moving the folder to the last page (Bliss launcher does not currently support creating new pages), making sure the page is not full. Then moving apps out of a folder works as expected.