I also have this problem, and why it exactly it happens seems to remain a mystery, but it it probably hardware-related. For me, it only seems to happen with a certain charger that is connected to about 40 to 55 V DC (E-Bike-Battery) with a 1/2 m long USB-C cable. This deliveres a very stable power supply, and it is probably a little known secret that you can use common “switching power supply phone chargers” also as DC-DC-converters with voltages lower than 100 V. However, different models show very different results, require a higher voltage and/or deliver less current.
Workaround is to cut the power (I installed an on-off-switch between power supply and charger anyway), operate the phone and turn it back on again.
When connected to a power bank with a 1 m long cable of the same brand, the “touchscreen over-reaction” does not seem to happen.