I posted a comment on an existing issue in Mozilla’s Bugzilla (2004150 - Dead Keys composition (e.g., ' + vowel) fails and characters disappear in specific web input fields (e.g., WhatsApp Web, Copilot)), since at first I thought it might be a bug in Firefox itself; then I saw the following issue reported here: VsCode keeps typing like the key has not been released and a linked issue in KDE’s Discussion: Last pressed key prompt leaks, getting stuck while switching tabs in VSCode or any VSCode clones, only in Wayland - Help - KDE Discuss.
I was able to reproduce both issues with Plasma Keyboard as the input method, but it works fine using IBus.
I could raise a bug in KDE’s bugzilla as looks like both issues are related to Plasma Keybord. However, still interesting that I cannot reproduce the issue with Chromium.
Edit: Reported upstream: https://bugs.kde.org/show_bug.cgi?id=518141