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If it does not count reliably, it suggests a problem with
the quadrature.

The count should be absolutely consistent.

Tommylights suggestion to use an oscilloscope is good.

Also if you can run the spindle at (very) low constant speed you can look
at the encoder A/B signals with halscope.

The signals to watch are:

hm2_7i92.0.encoder.00.input-a
hm2_7i92.0.encoder.00.input-b
hm2_7i92.0.encoder.00.input-index

( assuming the spindle encoder is encoder 00 )