Sony A7 IV A7IV switching from M(anual) mode to A(perture) mode accidently

[photo_enthusiast]

I have a 3 months old Sony A7IV and I noticed that in manual mode (aperture, shutter speed and ISO are fixed), the ISO is jumping to 10000-16000 every now and then. I am surprised since my ISO range is set to 100-3200. Same happens with shutter speed, that also changing, though the dial is set to M. This happens when I am in indoor taking a dance concert event photos. I am forced to switch off and then switch on the camera to get my original settings back (ISO 640, A - 2.8, S - 1/200). This had happened in 3 events so far. And found that issue was happening for both Sony 1.8 and Tamron 28-75mm f/2.8 Di III RXD Lens for Sony E. Wondering if this has been a problem with anyone else and if there is a solution available for this issue. I am on latest firmware (1.11).
Thanks in advance.