So you didn’t include any relevant information about your hardware setup here but I’m going to use the assumption that you are using some sort of grblHAL device.
It’s possible that you have an event linked in your firmware configuration to a specific AUX pin for the spindle on trigger (M3/M4)
If you’re using SienciHAL (SLB or SLB-EXT without having flashed a more recent version of grblHAL):
You need to check that whatever port your ATC switch release is set to does not trigger on M3/M4. This is configured by changing $456-$459, which each directly map a trigger to a specific aux (Aux 0, 1, 2, 3 in order) directly.
You’ll probably have to set the output to M64-65 only option to act as “disabled” for SienciHAL.
If you’re using a more modern version of the grblHAL firmware
You’d need to verify your Event Trigger Source Selection ($750-759) to see if any are set to activate on spindle enable (event 1).
At that point you’d need to update the related port assignment to either activate/stop activating the aux pin. Event always directly correlate to the port assignment (So $750 triggers $760, $751 triggers $761 etc).