I am also experiencing this issue, it is the easiest to trigger by manually removing chunks in the described way but this bug can also be encountered in normal gameplay, if the chunks are deleted or corrupted for example as a result of a crash.
I think it would be best if there was a system that checks if the newly generated chunk will smoothly fit with chunks next to it and if not to engage blending in that area.
It blends them properly only if the new edge chunks have their InhabitedTime, Timestamp and LastUpdate set to 0, if not and they have been visited before it won't blend them, I don't know if it warrants a bug report.
Regardless I think that this issue should be addressed as server administrators often remove chunks after they are generated, sometimes player maliciously generates hundreds of thousands of chunks that have to be removed to decrease the ridiculous world size, sometimes they build offensive stuff or lag machines that need to be deleted and if they do it on blended chunks after the upgrade, there should be an option to re-enable blending in that region.
Even if you don't want to support such use cases, chunk deletion can happen in normal gameplay like I mentioned above and as is evident by the chunk deletion issue preasent in Pre-Release 5 so I think this issue should be resolved regardless.