G28 Fine, then G29 ABL Probe hits bed

[csader]

Does it report all of the end stops and probes you think you have? Are the reported values correct? Are the values correct when you use your finger to trigger a particular end stop or probe?

Yes, yes, and yes.

Once that is all correct, I would do a G28. And then I would follow it with a M114 to ask where it thinks the nozzle is. Is the value reported reasonable?

Yes, seems reasonable.

I went back into my firmware and re-ran the instructions from Zennmaster, followed by an M502 M500 pair, just in case. No luck.

I went back into my firmware and re-set the Z-offset to Clough's default, followed by an M502 M500 pair. No luck.

Using Clough's calculator, I'm still getting that I should set the Z-Offset to 15.1-ish, which seems reasonable given the extra length of the probe arm I have installed. But when I run a G28, it runs fine. When I then do a G29, it lowers the nozzle height too low for the probe arm to extend.

I'm clueless at this point.