You might want to try Molly instead, which is a hardened Signal fork, it is really good and I use it myself for years already.
https://molly.im/
Because it still is Signal, you can connect with people that are on the Signal app also. (Like iPhone-Signal users)
As for the issue; I haven't got a clue what it could be, but I know I didn't experience issues like these on Molly myself.
If the problem persists, it might be a different problem than the app itself, like the size of the group or the cellular strenght? (I am no expert though, merely a user)