@cadu Checked the effector under the scope - solder looked good except for fiber microparticle debris and some surface oxidation that was causing the initial inconsistent probe readings. Reinstalled the effector reporting errors and it performed as expected, with 3 45-point mesh probes completing without "inconsistent probe reading" error and without the double parabolic shape.
G29
45 points probed, min error -0.165, max error 0.116, mean 0.001, deviation 0.057, with all 3 sets within 0.006 of those readings
My best estimate atm is that the increased speeds, acceleration, and short high-g movements in the input shaping tests after updating the firmware created the hidden hairspray/microparticle bead on the hidden idler pulley. The parabolic bed shape completing without reporting errors was probably the intersection of probing coordinates and the belt bumping over the bead. Prints didn't show the error due to the longer movements flattening the bead.