> I don't remember any reports back when 2.4 was released about this.

At least here on MorphZone there were reports back then:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=4991&forum=9&start=7

> the pink background is the transparent colour, so somewhere in the system this is not handled correctly.

The above-linked thread also has discussion and workarounds of this.