P6K Aperture Reading Not Displayed

I was shooting at the weekend with one of my P6Ks and a Sigma 18-35mm f1.8 lens.

I noticed a couple of times that the aperture setting reading had stopped being displayed on the camera's screen and on an external HDMI monitor. The first time I switched the camera off, took off the lens and refitted it. When I switched the camera back on the aperture setting was displayed again.

A while later I noticed it had gone again. This time I tapped the screen over where the aperture setting should have been displayed and it came back.

Has anyone experienced this?

I've had this issue with the Sigma lenses in general. In fact, with the Sigma lenses on the Canon 7Di I had the issue where I lost aperture control and focus indicators. The Sigma 18-35 and 50-100 both have the issue. I've lost aperture control on both the lenses on the UM4.6K, UMPG2, and the Pocket 6Ks. It's definitely an issue with the Sigma lenses. Or it could be the crappy EF mount. There's a reason I prefer PL with proper cinema lenses.

Must be the mount when not contacting properly.
My only Sigma, the 18-35, doesn't do it on the UMP 4.6K G1, but maybe I'm just lucky.

I have had it happen once or twice with the Sigma Art 18-35 on my Pocket 4K, and just reseated the lens in the Speedbooster.