As a brewmaster monk, the stagger incoming damage does not appear to be merged. Every tick of damage from the combat log is appearing in the incoming field unfiltered.

I tried to look at the configurations but I couldn't find any other class spell incoming damage that is spam merged.

The ID 124255 is correct from what I can tell in the combat log. It's also the ID that xCT seems to be seeing (enabled the show spell id setting)

Version of xCT+ is 4.3.5-beta5

This was fixed with #185 and 718a38a

Here is a short log with advanced combat logging, I don't know if it might contain some more information about how stagger damage is logged.
WoWCombatLog-060721_133441.txt

I just realised that the stagger does show up merged in the outgoing window. Do xCT have some default filters, because I feel pretty confident that noone wants to see stagger in their outgoing. Of course I can filter it on my own, but feels like it'd be a pretty safe defaults