G28 Z; ignores endstop (2 replies)

And before chorus of groans begins:

Z endstop:
-Z-min end stop function *verified* via M119 and manual actuation (*triggered*/*open* on manual actuation)
-Z-min axis end stop switch *verified* functional by testing against X axis (homing works on X, Y)

Behavior:
issued (via serial port) and z axis promptly crashes into table despite limit switch being actuated.

Things tried:
-different switches
-verification of switch operation using meter/oscilloscope
-Z-min, Z-max switches (manually actuated during G28 evolution)
-switch in question used on X axis (temporarily wired, manual actuation of Z switch for X axis, does trigger complete homing behavior)
-M119 verification

I've also poured over the documentation regarding endstops, probing, and config settings, I'm not seeing what might cause it
( #define Z_MIN_PROBE_USES_Z_MIN_ENDSTOP_PIN) is enabled but that's the default.

My configuration file has only been minimally changed to select for ramps 1.4.


Marlin version: 1.1.9
hardware: ramps 1.4/mega2560 controller
printer: custom