Hi,
I just discovered this same problem on a SOTA activation last weekend. I had been using EFLW (41 feet) via 9:1 UnUn to a KX2 , no problems tuning on bands 10-40m. I switched to using my SW-3B and maAT-10 tuner. No problem on 20m or 30m , tunes quickly. I tried 40m and as soon as I TX within a second the SW-3B stops transmitting so cannot tune wire. I added wire to 58 feet but same result. At first I suspected maAT-10 but checked at home on my 40m dipole and vertical and the SW-3B/maAT-10 combo works fine on 40m. So, I suspect RF from the EFLW getting back in to the radio is the issue. I have added several clip on ferrites to the RG174 patch leads between the 9:1 Unun, maAT-10 and SW-3B and will try it out on a summit next weekend and hopefully this will allow 40m operation. Will post results .
Declan