Merge Request gets stuck on Checking pipeline status when using $CI_MERGE_REQUEST_LABELS

Adding

workflow:
    rules:
        - if: $CI_MERGE_REQUEST_LABELS =~ /skip ci/
          when: never
        - when: always

causes the MR to get stuck on Checking pipeline status indefinitely. Changing it to

workflow:
    rules:
-        - if: $CI_MERGE_REQUEST_LABELS =~ /skip ci/
+        - if: $CI_COMMIT_MESSAGE =~ /skip ci/
          when: never
        - when: always

for example, gets it unstuck.

I also tried to add those rules directly to the jobs with the same result. Why does it stops working when using the documented CI_MERGE_REQUEST_LABELS variable?