UR-RT4 (or UR44) - iPad - Cubasis : noise issue

I recently bought a UR-RT4 interface an iPad 2008 9,7 inches and Cubasis last version.
I have noise issues when I plug a guitar in Hi-Z 1 or 2 input (I tried an electric fender stratocaster, an electro-acoustic Takamine TSF40C)
while the interface is plugged in the iPad with lightning - usb/lightning adapter (apple one, not a fake brand).
If I touch the interface (or the strings for the Takamine), the noise decrease a bit, so it seems to be some ground noise issue.
As I thought it was the interface, I ordered a UR-44 interface to replace the first one : the result was exactly the same ! So I felt a bit of a disappointment…
I did a lot of tests, different places, different cables, guitars, firmware update (UR44).
The sound vanishes only if I plug the interface in my desktop with usb cable : the desktop ground seems to make a difference.
I tried Thomann support and Steinberg support but they failed to help me as the UR-RT4 (or UR44) seem to work fine (almost unfortunately…).
Steinberg support thinks that it’s an app issue and they don’t support app, so here I am !
I didn’t find a lot a similar issue on the web, except this one : https://www.gearslutz.com/board/music-computers/935456-steinberg-ur44.html.

Any idea would be much appreciated ! :wink:

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