Initially reported there: Unvanquished/Unvanquished#782
But the thread reported two issues, this one if for the blending issue.

Here is an example of that issue (taken from Unvanquished/Unvanquished#782 thread):

internal:

external:

The depicted map is the Terminus map.

Released maps usually don't reproduce the bug since we usually use external lightmaps as they compress better. This issue may still bite us in the back later, and we can't assume any Dæmon-based game will repackage every legacy map available on this world, and maybe some may not have the right to.