The previous fix loaded the pixels so existing tiles were not overwritten.
However the Cycles render buffer is expected to be scaled by the number of
sample, which was not taken into account.

This is not ideal in that previews could have a mismatched number of samples
between multiple objects, though the result will be correct. The better solution
would be to bake all objects together per tile, rather than one after the other.
But that is a bigger change than we can do in 2.90.