compileGroovy annotationProcessing warning

I have a Gradle project with many(~15) subprojects that is actually a project of customs plugins.
The projects are written mainly in Java, but 2 of them have also groovy code.
I am using Gradle 5.6.4 and everything works perfect.

I am trying to upgrade to Gradle 6.7 (tried also with 6.6.1 and got same issue)
and I keep getting the following warning only on the 2 projects that have groovy code and only on the compileGroovy task.

The warning i get is:

> Task :myPlugin:compileGroovy
warning: Implicitly compiled files were not subject to annotation processing.
  Use -implicit to specify a policy for implicit compilation.
1 warning

I guess my questions are:
What does it mean?
Why suddenly in Gradle 6 and not 5.6.4?
What are the implications?
What is needed to solve it?