@crchisholm, I had this exact problem, I have a v3 as well and for months this was driving me crazy. sometimes it would self test fine and then others times not.
before powering up I would pull the pin all the way down. and then sometimes that would help assist with a successful test. Other times unplug and replug power over and over until it would self-test. This weekend I had the idea that maybe the set screw for the pin height wasn't down enough. even though I calibrated according to the documentation. So tonight I turn screw down. in between each screw turn testing with power off/on. no luck. a little bit more still not. a little bit more and dada it works every time! going to run with this and hopefully it's cured! Ah nope, worked until I had to home the Z axis. so move set screw around more and still not self-testing.
resetting M280 P0 S60 and the M280 P0 S10 to test. blinking... voltage is good. 5.2v.

okay I have spare PIN... put in. self-test fine. run M280 P0 S120 works good. run again. works good. run again. works good. M280 P0 S10 deploy perfect M280 P0 S90 retract perfect. Z Home perfect!

Wondering if the magnet lost some flux...