I run several Bitcoin nodes on Amazon EC2 and notice some of them stop making feeler connections after a while.
I checked the debug.log of the nodes and see that "Potential stale tip detected, will try using extra outbound peer" messages are printed out frequently, up to 10+ per day.
The messages appear more frequently when the feeler connections disappear.

I expect the nodes still make feeler connections in between events of the 9th outgoing connection made because of the stale tip.
I presume SetTryNewOutboundPeer() incorrectly set the flags somewhere so that fFeeler is always false.

The problem happens naturally for some nodes after running 20-30 days.
All nodes are running Bitcoin core 0.17.0 by default non-stop on Ubuntu 18.04 LTS and with some minimal logging messages I added myself.
I tried to restart one node and the problem is gone. I guess because outgoing peers have been chosen again when node restarts.