Mark sorry to hear about your loss. We have another jet pilot in our club who lost his JL F4 about a month ago. He has flown this model extensively on Spektrum gear. Went the same route as you and tried it in a foamy first, where it performed just fine.
Then commtted his F4 to the DX18 and it went in. It range checked OK before flight. Weird!
I have heard of a F3 glider pilot having a similar problem but I have not details yet.
John