The following MNWE uses \dodoubleemptywithset. In ConTeXt MKIV it works as expected,  In ConTeXt LMTX an error results.

\starttext
\unexpanded\def\doMyDefs[#1][#2]{\getparameters[MD#1][#2]}
\def\MyDefs{\dodoubleemptywithset\doMyDefs}
\MyDefs[A,B][key=val,k=v]
\MyDefs[C]
\typeTEX{\MDAkey} is \MDAkey. \typeTEX{\MDBkey} is \MDBkey.
\typeTEX{\MDAk} is \MDAk. \typeTEX{\MDBk} is \MDBk.
\stoptext

Possibly, \dodoubleemptywithset is not used anymore in LMTX, in which case it should be removed. Otherwise this could be a bug. Or maybe I use it wrongly?

Cheers,

Christoph