Preface: I have submitted this issue to support at Harrison.
I decided to put my Reaper setup aside for a while and learn Harrison Mixbus. When I change the buffers setting in Mixbus 32C it doesn't change in the Axe I/0. If fact I never use 2048 buffers but the Axe I/0 is often set at that when I launch Mixbus. Since I'm playing some VST's that require 64 or 128 buffers to track in the DAW (Superior Drummer 3 and AT5) I change the buffers in Mixbus but it doesn't change in the Axe I/O Solo control panel and the buffers control is greyed out.
At this point the only thing that will change the buffer settings is to close Mixbus and launch Cantabile, my live VST host app. What's odd is when I set it to 128 in Cantabile I always get 256 in the Axe I/O control panel after I load Mixbus 32C which is low enough for AT5 so I run with that. Whatever I set in Cantabile for buffers reflects correctly in the Axe I/O so it's definitely something odd going on with Mixbus