Discrepancies in numbers of tests reported

We are seeing discrepancies between the number of tests reported in TeamCity and the actual JUnit reports.

The screenshot below shows a different number of tests reported for two builds with no changes between them.

The actual value reported by JUnit was 12197 in both cases.

In addition the build configuration runs two Gradle tasks as part of a single build step, the second of which reports 66 tests in JUnit, but these are not reported at all by TeamCity.

We are using TeamCity Professional 2017.1.2 (build 46812) with Gradle to run the build with Dockerized agents.