Yes it is the normal file extension for Scala source files. 

I noticed that the info.plist file for TextMate contains entries for .c, .java, etc.  but does not contain an entry for .scala

(Not surprising since it is not as common a language).

Using QL for  files that are listed in the info.plist file works for me so perhaps this is just a known limitation.

Sounds like your issue on Catalina is something different.