Hello Laszlo,
This seems related to this topic.
Basically, JaCoCo currently reports invalid coverage lines for Kotlin in some cases, and the analyzer cannot import coverage information for files impacted by this problem.
If you see in the logs error messages similar to:
Cannot import coverage information for file 'xyz', coverage data is invalid. Error: ..., this means it is indeed related to this issue.
Once this is fixed in JaCoCo, coverage information should then be correct when imported in 
.
Best,
-Chris