TL;DR: Follow the following GitHub comment.
The following applies to FG3 and sub-sequence versions if there are no fixes regarding this issue.
Setting the property mixin.env.disableRefMap is not a proper fix, it may work for projects *with the same mappings*, as you are expecting that the original source of the mixin files to be in the same mappings as your development environment, which is false for some projects. (And is extremely unstable, will even break if you have a different MCP version.)
Specifying mixin.env.refMapRemappingFile or providing net.minecraftforge.gradle.GradleStart.srg.srg-mcp may be an alternative fix.