For got to answer your first comment:
> Is this a bug in ‘texlive-union’?

I don't think this is related to texlive-union, but since I don't fully
understand the issue, I don't know if texlive-union could be tweaked to fix
this.

> Does it take time and space to regenerate the fonts?

Relatively little: < 5 seconds I'd say, less than 1MB.

-- 
Pierre Neidhardt
https://ambrevar.xyz/