We ran into a problem recently when trying to send SimpleX links on Lemmy. No matter what if a link was sent the recipient would get an error message instead of being invited to the room. After some trial and error we figured out that the QR code was the only way to send someone a working SimpleX invitation on Lemmy. If you send a link or post one something on Lemmy causes it to never resolve to the correct address and appear that the room is gone.

I'm not sure exactly what's going wrong but if you're having trouble sending links to others or having problems joining rooms from links on Lemmy give the QR code a try before you give up.