Homepod mini wandering stereo image

I have two homepod minis, set up as a stereo pair. They are on my desk, equidistant from me, and both equidistant from the router. The stereo image created is very strange and seems to wander from left to right and back every few minutes. The shift is not sudden and when the image starts to move it takes a few seconds to wander across. It is almost as if there is a very slight change of phase between the two speakers and for a minute or so the sound from one of the speakers minutely lags the sound from the other. The effect is only noticeable (and really quite annoying) when sitting at the desk - at a distance its imperceptible.

Anyone else experiencing this? Solution?