Thanks for the info re: wear patterns.

No, the droppage (both to the inside of the 34 as well as outside of the 50) happens not during shifting, but only under high power load.

My mechanic (when we were doing intial troubleshooting about the issue) said the FD looked fine, and double checked the limit screws and cable tension.

The 50 ring is slightly warped, which I think may have happened a long time ago when, unknown to me, 3 (!!) chainring bolts had backed out, and was only brought to my attention by a mechanic (we looked at it together, so I know he wasn't screwing around with me). I had just started riding a few months earlier, so it's very possible that I wasn't able to assess the resultant flex caused by the missing bolts at the time. Anyway, he replaced the bolts, secured them with loctite, and I've ridden these rings for about 15k miles (probably about 12-13k in their slightly warped condition).