How to stop capacitive buttons being triggered by acrylic sheet
The acrylic sheet can present a large DC offset, caused by the stored static charge on its surface. I suspect that charge is being detected by your port,  masking the touch signal itself.
To ignore that charge, you need to filter out DC and only pass AC.
I would suggest this approach...
Excite one side of the switch (the transmitter half) with a high frequency square wave, and the other side (the receiver half)
should connected to a high pass filter, to attenuate any lower frequencies / DC, so that only the high frequency component is passed to the port.
So, between the sensing element and the arduino, have a couple of stages of suitably tailored high pass RC filters, perhaps feeding a CMOS inverter to act as a buffer, then feed that filtered signal to your port.