OK, a few more data points for anyone still following or if you landed here via Google. I recently picked up a Yaesu FT-897 and took it out for its maiden POTA voyage. It has an FC-30 tuner attached to it as well. Running off of a EcoWorthy LifePO4 20AH battery. 2hours, 25 QSO's all over the country on 20m. All park to park. Even managed one contact on 10m, despite the poor conditions. Strong 59 signals from most stations. The difference in a 20W QRP radio and this 100w beast were noticeable.
Interestingly enough, the FT-897 tuned the JCP12 on 10m with absolutely no problem. It also tuned on 20m, no problem. But it would not tune it on 40m. Interesting. The G90 will tune the JCP12 on 40m no problem. So it looks like different radios work differently....who woulda thought?
So it appears that there is nothing wrong with the JCP12 antenna. I'm chalking up the G90's inability to tune on 10m due to the fact that the radio's head unit is separated. Something about that, I'm guessing, and the close proximity of the antenna to the radio in a POTA setting is throwing it off. I've read a lot of reports about the front end of that radio being sensitive. At least that's my uneducated assumption, given that the FT-897 (a 20+ year old radio with a 3:1 tuner) will tune it on 10m, no problem.
So it appears that if I want to hunt or activate on 10m, I'll either need an EFHW with the G90 or I'll have to take the FT-897 out. Data points all around. I'm beginning to feel confident in this POTA that. Diamond Hunter now, with 4 activations under my belt. Ooorah.