This situation occurs if a promise is prefixed with a "class_exp_1::" notation and has a "ifvarclass => "class_exp_2" statement. Then the log message says:
Skipping next promise '<promise>', as context 'class_exp_1' is not relevant
Whereas it should say:
Skipping next promise '<promise>', as var-context 'class_exp_2' is not relevant
This has no undesired consequence on the actual actions CFEngine performs but it does make debugging harder. This is new in CFEngine 3.5 and does not occur in 3.4.x.
I have found the issue and a Pull Request is coming.