I am seeing the same thing in the Finder on Catalina. My output is the same:
🦊 qlmanage -m plugins | grep TextMate
public.source-code -> /Applications/TextMate.app/Contents/Library/QuickLook/TextMateQL.qlgenerator (1.0)
public.text -> /Applications/TextMate.app/Contents/Library/QuickLook/TextMateQL.qlgenerator (1.0)
I was not aware of the qlmanage utility until I saw this email....
I am running MacOS 10.14.6 (Mojave) and when I use QuickLook to view a source file such as Foo.scala I just get the generic QuickLook window describing the file.
But if I use this command: qlmanage -p -c public.source-code U.scala
Then I get the correct QuickLook window that uses the TextMate theme.
It seems that the Finder is not associating the "public.source-code" type with the file. However I have told Finder to open all .scala files with TextMate and indeed it is listed as the default App for opening the files. Not sure if there is something else that must be done so that Finder knows that this is a source code file?