Yes -seems to be very closely related.
However, at least one of the slaves on which the problem was manifesting for us had no pipeline jobs running on it at all.
However, I did notice the following:
Using test files which previously failed to be parsed (causing above error) - and using the same jenkins job and same jenkins slave:
Uncheck the "Stop and set the build to 'failed' status if there are errors when processing a result file*"* option (or set to false in the pipeline) then run the job, and it passes okay.
Recheck the "Stop and set the build to 'failed' status if there are errors when processing a result file*"* option again, and this time the nunit parser works okay.
Not sure if this work around is always going to be replicable or if I made some other manipulation somewhere along the way that explains the fix - I'm going to try with our latest test result files (when the full integration build has finished) to see if I can get a failure.