[otfautohint] will not hint if zones are too close

[frankrolf]

I got a report

afdko.otfautohint.fdTools.FontInfoParseError: In FDDict FDArray[0]. The top of zone AscenderHeight at 698 is within the min 
separation limit (3 units) of zone FigHeight with the bottom at 700.

Reading the T1 Specification (page 37, chapter 5), I confirm that otfautohint is correct:

This is confusing since psautohint (and autohint before that) did not enforce this (I learned that alignment zones need to have a minimum distance of 1 unit + BlueFuzz). Especially in UFOs which have previously been edited with psautohint, we now produce a fragmentation of hinted (untouched) and unhinted (touched and their dependent components) glyphs - which only surfaces once we run makeotf (lots of “unhinted” output, but without an explanation as to why).

• What tests need to be conducted to understand if the zone distance 3 really is crucial to rasterization?
• Might there have been an update to the spec where this distance was decreased? (might well be that autohint got it wrong in the first place)