Thank you @locke496
You were right to insist on checking the logs! Those Create input context failed: Operation was cancelled errors from ibus were actually the culprit.
I ran ibus exit and immediately tested Ptyxis - backspace repeat works perfectly now!
Switched to fcitx5 as my input method (I need Japanese input anyway) and everything is working fine. The issue was ibus conflicting with GTK4 apps on runtime 49.
Thanks for your help and patience!