Good tests, I think you’re in the realm of bed tramming (manual level) now.
If you’re REALLY avoiding the tram process:
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Try rotating the plate (like, clockwise 90, see if the bald patch travels with the rotate) and/or using a different plate to see if the problem is a banana plate or the whole tram that needs it
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Back in the day, we used to put layers of masking tape under the plate to add 0.25mm layers of thickness and resolve minor bed mesh unlevel problems
If you’ve only got one plate that you use… You might be able to get away with some quick and dirty shenanigans by doing the same. See the following horrible hacked together example of lifting +0.50 in a corner…
Good luck!