The TagsTest relies on priority takeover to have certain nodes take over as primary. This makes the initial startup and configuration phase unstable, since it can cause an existing, lower priority primary to step down while another node is syncing from it. This was fixed by SERVER-36327 in 4.0 and beyond by using the stepUp command. However, the backport for v3.6 was denied (BACKPORT-2925) because pv0 does not support the stepUp command so it was a bit more non-trivial.

The BF on v3.6 doesn't happen often and this is probably low priority since v3.6 EOL is soon.