Really occasional malfunction of the Schmitt-trigger input
Your awareness of stray ISR resets, tells me you have an EMI problem.

It can be caused by stray E-fields, modulated by your hand that acts as an antenna and variable capacitor to couple line E fields near a floating supply voltage used on MCU.

Although 0.1uF seems low impedance, the thin wires also raise ground impedance, so the distance to MCU can be important and RF broadcast signals can also combine with this interference.  Often Common MOde (CM) noise converts to Dff. Mode (DM) noise by unbalanced impedance of signal and reference. (supply)

To reduce stray E-fields; typically, ground planes, shielding , ferrite beads, twisted pair wires, using tighter/smaller layouts to reduce loop area are some methods used and use lower impedance sensors.