The string for the mod name prefix colour is generating ยง instead of § for the values in the config.
It is generating S:waila.cfg.nameprefix=ยง9ยงo when it should be S:waila.cfg.nameprefix=§9§o.

Using the correct symbol when modifying the config works - loading the instance up with the string set to §e prints the mod name with yellow formatting - but it replaces the string with ยงe during load.

Not fixed yet :p

*tongue face reaction*

The configuration is supposed to generate as \u00a7 instead of the actual section symbol. I will properly escape the unicode.