@mfalkvidd Hello! I am one of the other "we" mentioned in this post. I don't have access to the layout files at this moment, but I checked the Gerber files and none of the SPI lines are routed under (or near) the antenna. They are also not routed near any other lines that route under the antenna.
"The placement of the nrf's antenna on the board is
very unusual." Bad
Yeah..... In our original goal/planning for this sensor board, we were going for a tight footprint that would sit inside a 3D printed enclosure easily and didn't think about the antenna being parallel to the PCB underneath it. Once we started having issues, we used a dremel to remove all the copper we could from the board below the antenna, but that didn't change the behavior we saw.
Technically, we did that dremel experiment on a different board (we made two different designs) that was showing the same issue. Below, you can see an image of that experiment.