The following example produces different results when processed by LMTX 
and MkIV. In particular, a space between the X and Y is not 
auto-inserted by LMTX, while it is by MkIV.

    \starttexdefinition TEST #1
       #1\autoinsertnextspace
    \stoptexdefinition
    \tt
    \starttext
       \TEST{X} \emph{Y}
    \stoptext

The MkIV result is the correct result (in my opinion).

-- 
Rik