I have some long existing Jenkins jobs with TAP tests configured. However at some update of either Jenkins or the tap-plugin the tap plugin stopped functioning correctly. This wasn't noticed earlier because the build jobs did not fail.
The plugin run ends with a message "Found matching files but did not find any TAP results"; in the workspace the testresult file is indeed empty. Analyzing this further it turns out that tap-plugin 1.7 works correctly, but any higher version seems to truncate the testresult file to 0 bytes before processing it.