When I add an attachment to a reply (or forward), then my reply email is just gone and I am back on the original email where I did tap on reply.
Completely deterministic every time. But only with more “complicated” emails I have received, e.g. from a hotel or rental car booking.
But no problem adding an attachment to the reply of the signup email of this forum. Adding attachments to a new email also works fine.
This is inconvenient since I want to forward booking emails regularly with additional attachments.