I have selected 55 meshes that make up a wooden structure in my level, which share only two materials, and hit the merge actors button to create a single mesh.
The resulting mesh has 55 material slots, am I wrong to expect only two?
Looks like this is a bug with the current version of the UT editor (based off 4.12 preview, not release).
I’ll have to wait for the next version for the fix 