Well, hate to admit this, but since it may help you and has an easy solution...
My front derailleur seized on me this summer. My LBS asked me if my electrolyte drink spilled, but I only ever ride with water, so that wasn't it. After a few additional questions we figured out that it was due to my sweat falling onto it and then drying. Was a really hot summer here in the mid-Atlantic area. A bit of hot water, followed by a good drying and some lubricant and I was back in business.
Might want to give that a try if you've not yet solved the problem.
Life's a journey--and I'm just here for the ride.