Unremovable white transparent rectangle on subtitle

[kev-mantissa]

Hello,

User on Fedora Linux 41 here.
Downloaded Aegisub from Flathub, so current version is 3.3.3.

Weird thing is from the get-go there is this unremovable white rectangle on the subtitle text, both for Dummy videos and real videos.

Managed to edit a video anyway, and it does not appear when using the .ass in external video media players, but for refining things on timing for example, it is not practical.

Any ideas what could be the issue? (Fedora-related? other issue?)
I ran an overview on the options and styling parameters, but without success.

Thanks