At this point I'm unsure I properly understand how this works and where the problem may lie (the sender, or the receiver, or both sides).
The article that Emmanuele Bassi (npub1ycl…36x4) mentioned at the beginning of this thread is supposed to explain/document things, but my comprehension of it is limited. If it's unclear to you too after reading it, then I guess Julian Sparber (npub16d8…0x8u) may be the only one who truly knows what needs to be done (and in which toolkits vs apps), as I'm getting pretty confused at this point 😅