Internal display goes black when (partially) opening/closing lid, but only if no external display is connected

Hi,

I have been struggling with a strange issue in my MBP2016 for a couple of days now. It seem to only occur when using the MBP without an external display connected to it: when touching / partially opening or closing the MBP's lid the internal display will go black, not immediately but at the latest after a minute or so. Just FYI: The MBP also seems to go to soft sleep soon after, which - I think - is expected if the screen turns off.

Naturally I was thinking that this might be a loose connection of the internal display cable to the mainboard. However, strangely the internal screen never goes black (even when touching and moving the lid) when an external display is connected. So, I assume that this is rather a software issue and not a hardware. I would greatly appreciate any help on this! Cannot work in the garden currently because of this :-)

Thanks in advance and best regards,

Felix