Less CSI packets captured if antennas move closer together

[fwfmc]

Hi, I am currently working on setup to capture CSI packets in a 3x3x30 MIMO system. I mounted the antennas on two 3D printed mounts (one for receivers/ one for transmitters) where the distance between antennas on the same mount is around 4cm (1.57inch). If those mounts are sufficiently far apart, I am able to collect CSI packets for the whole 3x3 MIMO system. As soon as the mounts come closer together, some packets are only recorded for 2 or less transmitters. The closer the antennas get, the higher is the amount of packets which are not received from all transmitters until the collection is completely shut down when the antenna mounts come together very close.
I could examine this behavior in the 2.4GHz as well as the 5GHz frequency domain for very similar distances.
I use two Thinkpad T400 laptop with six similar, omnidirectional dual band antennas.

Has anyone encountered something similar and has a solution or an explanation for this behavior?

Appreciate any input and thoughts on this.
Thanks!