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https://phillipreeve.net/blog/review-tamron-24mm-f-2-8-di-iii-osd-m12/

It looks like the only relevant defects are vignetting and distortion, but the former is not a problem for macro, the latter not a problem for nature macro and landscape... At the moment I think I'll pick the 20mm, but these reviews are really make me wanting to have one of these latter Tamron lenses sooner...

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He also mentions issues with AF (but for macro I think is not so relevant), even though he says manual focus with this lens is not easy. Curious about that.

[Brightcolours]

When you have mechanical manual focus, you can use that instead of AF if you want to , for macro stuff. But when you have focus by wire that is not excellently implemented, manual focus is no joy at all. So yeah, a reason to skip suck a lens if it has AF issues (or is it just slow?) and not so good manual focus by wire.
And you say you are curious about that, but he lays out the issues pretty clearly:

1. Manual focus is non linear
   
2. focus angle depends also on the speed with which you turn the focus ring ("Like most I think that this behavior makes focusing manually harder")
   
3. Another issue is that sometimes the focus change isn’t continuous but happens in jumps
   
4. Add the small lag experienced with all focus by wire lenses and overall I found the Tamron to be harder to focus manually than most other E-mount lenses.
   

All this means that you keep overshooting focus and that it will be tedious to shoot manual.

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I am curious because this defect is so blatant, but other reviewers didn't report it. Could it be a copy defect? Related to hardware or firmware? In the latter case it might be fixed by an update.

Actually all the lenses that I'm using for macro work are mechanically focused (with focusing helicoid, mechanical of course). If I have to use manual focus for macro, it must be mechanical or by wire, but very well implemented. I've never used autofocus in this context, it might help; being slow is not a problem.