How to fix Zbrush Udim displacement seams
keyframe
Or How to get proper Zbrush displacement from Mari udim texture workflow.
This seems (har har) to be a persistent problem for a good while now, and after pulling my hair out troubleshooting and testing online solutions for my own project for the last couple days, I was able to figure out what was happening:
zbrush will double up the displacement of the points on the shared edge of udim tiles (sounds so obvious now).
To fix this behavior, all we need to do is contract the visible points once (ctrl+shift+s) before applying the displacement on an adjacent tile that already has full displacement. Note that masking is completely ignored when working through the displacement tab, so it has to be done through visibility.
Now, if you have lots of udim tiles and depending on how the seams are cut; it will require some juggling and some zbrush hide/unhide gymnastics to remember which udims have full displacement and which have been contracted, so it's a less than ideal solution for very complex assets, but at least it's fixable...zbrush is a harsh mistress sometimes 
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Edit:
Come to think of it, it could also probably be fixed with morph target and negative layer stacking. This method might be simpler for really complex assets.
I'll have to test this out...
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Still though, the first method is much cleaner.