Ah sorry guys!
I've included this change into one of previous versions, but @voldapet stopped me because he had some troubles in compatibility with Locus internal themes. And then I've forget on this topic, sorry.
As I think about it, best should be add extra parameter to keep current system compatible with old and new themes.
So in next version of Locus Map will themes support parameter "scale-line-dy-by-zoom" in header of themes. So will be able to add aditional argument scale-line-dy-by-zoom="1" , which enable this required scaling. Possible? Let me know how it works in next Beta version (probably tomorrow ).
So example of such locus internal theme header:
<?xml version="1.0" encoding="UTF-8"?>