For future reference, I will archive the email thread we had about this issue:

@WMDE-Fisch

[…] regression, that we introduced (it seems) with the first version but no one complained about it (so far).
should be: https://wmde-wikidiff2-unpatched.wmflabs.org/core/index.php?title=OneWordTest&action=historysubmit&type=revision&diff=316094&oldid=316093
is: https://wmde-wikidiff2-patched.wmflabs.org/core/index.php?title=OneWordTest&action=historysubmit&type=revision&diff=316094&oldid=316093

@jkroll

on first glance, this seems to be what's happening: The amount of change between lines is above the threshold, so the change gets converted to an add plus delete. This is intentional, we added this so that the change detection can kick in.
I'm just making sure I understand what's the bug here. Should this be re-merged as one changed line? That does happen when a moved paragraph is detected with a target that immediately follows, but not in this case, probably because the amount of changed characters is 100%.
It surely looks weird if you compare it with the output before, but what should be done here if we 'fix' it? Add a special case when the change is only one word, regardless of the character count?

@jkroll

When a diff only contains one single change, it makes no sense to convert a change to an add plus delete. We could add a special case for that. Could be an easy fix for this.