The problem here was that the "frame_start" and "frame_end" RNA properties of
the Stepped FModifier were shadowing/overriding "frame_start" and "frame_end"
properties of the base FModifier. As a result, when the range() callback
for the In/Out parameters (defined as part of the base FModifier) checked
it's start/end properties, they were always still zero, meaning that the
acceptable range for the In/Out parameters was 0 -> 0 = 0.
Note:
If you've got old files with this problem, you'll need to manually click on
the frame_start/end properties to flush out the old values. It's probably
not worth the effort of applying a version patch for this (given that this
modifier is not one of the most often used ones AFAIK).
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