Touchscreen not working properly when cell phone is on your back?

I recently got the Huawei mate 20 lite because my old phone broke.

This works quite well, but I have a problem that I can't explain:

Whenever my cell phone is on my back, for example on a table, the touchscreen works very poorly and you have to press several times until an action comes through.

Can someone help me there?

If the part is used without a case, it may not lie "flat" or lie flat due to the slightly protruding camera unit; thus the touch tends to recognition problems, there are also other brands with this little "shortcoming".