Well, I'm stumped.

I was auto-bed-leveling like a champ with Clough42's Itty Bitty Dual extruder, but recently I put aside that excellent assembly and installed a single extruder with his ABL servo. I replaced the firmware with beckdac/Marlin from github yesterday, and after changing my z_probe_offset_from_extruder to -4.87, servo angles (0,0 0,0 61,155) and estop = 915, I thought I was golden... but for some reason, I get a different Z height reported with G28 (5.0) than I do in the same spot with G29 (~3.9).

Furthermore, the LCD is reporting nearly the same heights (as expected) on 1-8 probes on the topological probing... but on the 9th/last probe, it goes from ~4 to 7.8! As a result, the hot end tries to dig a trench into the bed on print.

If I simply print with G28, and omit the following G29 command, the first layer print is as expected, but obviously I have no ABL routine.

I've disabled EEPROM settings, and I can't think of anything else to do. I've been up and down the Configuration.h file ... but even more puzzling, when I flashed back to Clough42's firmware (which was working well for several weeks) the printer seems to exhibit the same bad behavior... erroneously high z-height report on the last probe, and a resulting trench run.

What am I missing?