I came across a similar issue when upgrading from Shibboleth 4 to 5 as well. The Attribute Resolver was just completely ignoring my custom data connector with no error message. The change I had to make was calling super.doParse in my BaseAttributeDefinitionParser. Shibboleth 4 was able to automatically pick up the custom schema without this, but Shibboleth requires the super method to be called.
There are some more information here: https://shibboleth.atlassian.net/wiki/spaces/IDP5/pages/3199512485/Developing+Attribute+Resolver+Extensions