TestReport URL randomly changes if there are multiple test results for tests with the same name.
How to reproduce:
1) create a job that imports 2 or more test results of the same test. The best results you will have if one of this test result failed and other were successful.
I'm using MSTest plugin and I'm importing *.trx files.
2) Import these test results and go see the output from failed test.
3) Refresh this url few times. You could notice that Jenkins from time to time change the output of test from the one that failed to the one that were successful and vice versa.

Please check attached video