When I type in the WhatsApp application (only that application) with the text replacement being active, the last syllables of the word gets duplicated. But I found out that this only happens when the "replacement bubble" appears on top of the message.
Here's a gif:
I tested almost everything I could find... Reseted NVRAM, disabled repetition, tried to uninstall any Wacom Software (as seen here), and I had none installed, but nothing works.
I'm running a MacBook Pro 2019 with Catalina Installed. And this only happens in this particular app.