To fix this, I had to add the @Order(-1) annotation (from this issue https://github.com/grails/grails-core/issues/11404) to my DoubleValueConverter class for it to have higher precedence than the default (0?). Also, this change was introduced in Grails 4.0.2, so I had to upgrade from 4.0.1. Hope this helped someone :)
This is a regression of #95 .
The solution is in: https://stackoverflow.com/a/65393044
Quoting: