Usually only fraction of parallel test runs have a status comment on Pull Request once complete

[ptbi]

We have 13 test groups running in parallel for each Pull Request. Sometimes all of the groups do comment on the Pull Request with the status once completed, but often only 6 or 10 of them end up commenting on the Pull Request and triggering a notification email. All of the groups do get triggered and the tests are run on each Pull Request every time as expected, so it's just the comment that often is forgotten by a subset of the groups. Is this a known issue?
Thank you