Just signed up to say you’re not alone. I’m not a Manjaro user.
I was a Ubuntu user for a few years, tried KDE recently, loved it and yesterday installed Kubuntu.
All was going well, until a few hours ago I noticed the same issue. It seems to only happen on Whatsapp Web, and only if there is an accent in the first letter. Any letter with any accent (I tried ´, ^, ~ and ¨). The problem also seems to happend if you type the accent followed by space, which normally would output the accent alone, but is instead deleted.
Tried changing fonts and languages, both in Firefox and in the System, but no deal.
Also made a video (this forum apparently blocks me from posting it in your post) (also blocks me from posting links):