This may apply to other tests in primary_only_service_test.cpp, but at least one of the tests (i.e., RecreateInstanceOnStepUp) may fail due to a race between the thread that is completing stepUp for a POS instance and another thread that attempts to create an opCtx on the service (see here):

try {
    auto opCtx = cc().makeOperationContext();
    ...
} catch (const DBException& e) {
    _documentWriteException.setError(e.toStatus());
    throw;
}

The operations are interrupted with NotWritablePrimary if they are created when the POS instance is still rebuilding. Adding the following line before making the OperationContext fixes the data-race, but may not be desirable for the actual fix:

try {
    AllowOpCtxWhenServiceRebuildingBlock allowOpCtxBlock(Client::getCurrent());
    auto opCtx = cc().makeOperationContext();
    ...
} catch (const DBException& e) {
    _documentWriteException.setError(e.toStatus());
    throw;
}

This ticket should propose a fix that ensures the operations are not interrupted, either by strictly ordering stepUp and construction of opCtx, or using the earlier suggestion.