My software update was done during mid-Dec. Others posting here had not seen an update yet. It could be the different models require different software and their fix is coming.
The software version did not change when the update was finished. Only the build number changed.
The software version on all 3 of mine is 11.5.0
The build number was 4235 before the build and 4312-CR after the December update.
I suggest that you check your build number and your update date. I had one Roku that did not update on its own. Not sure why. It is the further from my router and does buffer a lot. I manually updated that one. You may have to manually update yours.