Not sure if this issue presents itself in Minecraft or just when using mods simply because the thousands separator is never used in Vanilla.

I'm using Swedish region settings which uses a space as thousands separator and it renders as a "NBSP"-sign instead.(See attached image)

EDIT: I was able to reproduce this in Vanilla too! (See additional screenshot!)
Btw, used this Java code to get the NBSP-signs:

NumberFormat nf = NumberFormat.getInstance();
System.out.println(nf.format(1000));
System.out.println(nf.format(100000));
System.out.println(nf.format(1000000000));