Anyone else have problems with JP stereo jack through wireless?

I recently picked up one of the new Boss WL-20L wireless units and was very impressed while playing through the mags output on my JP6 (circa 2014, so I believe it has the newer style preamp already). So then I put it through the stereo/piezo output and there is a noticeable high pitch frequency coming through. Doesn’t matter whether I select piezo only, mix, or mags only… everything coming out of that jack has a high pitch artifact when playing (somewhere up around 10k or higher). It’s not feedback or interference – it’s the same high pitch whine no matter what note is played and it never changes frequency. If I swap it back to the mono/magnetic jack everything sounds perfect again.

I assumed maybe the 20L higher impedance model might just be the wrong choice with the EBMM preamps, so I went back to my local shop with my JP6 this time and tried the standard WL-20… same problem. Tried into different amps and modelers and also tried the comparable Line6 wireless system… same issue. Only common denominator was my JP6 BFR. Again, this doesn’t feel like an interference issue. The frequency of the whine remained constant across all tests.

Could there be something up within my preamp that isnt grounding properly with that wireless jack inserted? The jacks on all of those newer rechargeable, auto-on transmitters have a little metallic pin that when compressed turns the unit on. Could that be doing something unexpected on the stereo output jack? Again, a physical cable works 100% fine. I’m at my wits end trying to find a wireless solution I can use with this. Really want to sort this out as this is my #1 go-to guitar that I plan on using 90% of the time live. I’d hate to do another preamp swap just to find out that that wasn’t the root cause.

Anyone else have any similar experiences or ideas???