I have not verified it but I may have discovered the solution. I was using a custom smtp server that forwarded my emails over the pstn as texts. This custom smtp server must not have checked for bcc recipients, which I have learned is how “send copy to self” is implemented currently.
See Why not copy to sent folder? - #6 by rkd
So no bug in delta chat, although I still propose the option to copy via imap as discribed in that thread.