Quoted from zaphX:

I did some more thinking and experimenting.
What do we know about the problem?
Fact: The only time it fails is when the VUK chucks it down the lane. Normal entry to the lane always works.
Fact: This issue is somehow wear related, as it doesn’t happen initially and develops over time.
Fact: The nature of the failure is not the gate, but a bounce up into the guard above it.
I’ve replaced the coil sleeve and can eliminate coil wear. So what else could wear here over time?
Hypothesis 1: The lane guide shifts itself out of position over time. I was able to disprove this one, as there really isn’t much play at all. Only a very, very tiny amount. Adjustments here were inconsequential.
Hypothesis 2: The guard over the Depths VUK bends itself upward over time, causing balls to exit airborne and bounce down the lane rather than rolling smoothly.
I bent the guard down a bit and so far things are looking good - I played for about an hour without any VUK drains.
Fingers crossed this puts it to bed.