My understanding is that the universe did not have enough time to thermalize before the epoch of recombination, so many patches of the sky were not in causal contact with each other, which means they should have different temperatures according to the original FLRW metric tensor.
How does this relate to the perfect blackbody radiation observed by Planck (2018)? Does GR predict something lumpy and inconsistent making it a less-than-perfect blackbody curve? It's my impression that you need inflation in order to make it a 'perfect' black body. Is that correct?