Used the following MS Test results file naming convention:
<Project-Test-Subfolder>\TestResults\<Project>TestResults-Build_${bamboo.buildNumber}-Rev_${bamboo.repository.revision.number}.trx

Which worked okay until we hit build #10, which should have passed. Found that the results parser was pull failures from older build 7 and bringing and failing the run.

My work-around is to reverse the rev and build variables in the file to:
<Project-Test-Subfolder>\TestResults\<Project>TestResults-Rev_${bamboo.repository.revision.number}-Build_${bamboo.buildNumber}.trx

a) Would make sense for the parser to key off of test results file's creation dates instead.
b) Would be nice if the timestamp variable was filename friendly (that's what I would have used instead to make Test Result file name unique.)