Got a reply last month here:
every second: Possible Tor bug, or possible attack if very frequent: Got 1 dropped cell on circ 17 (in state CONFLUX_LINKED None; old state CONFLUX_UNLINKED None) · Issue #95 · mikeperry-tor/vanguards · GitHub
Vanguards specific issue with Tor 0.4.8.9:
Connections drop on Tor 0.4.8.9