Hi,
 
It seems that the Lettrine module is one of those affected:
 
https://tex.stackexchange.com/questions/562534/does-context-fail-in-the-lmtx-environment-with-the-lettrine-module?noredirect=1#comment1418902_562534
 
But, honestly, first, I don't know what means "Of course it might have other side effects once in lmtx we everywhere expect freezing to be enabled."
I suppose that, in order not to be a nuisance (and not to digress into this real subject of interest), a short answer would be enough to put me on track to investigate it.
 
On the other hand, I suppose the best practice, from what I have just read, would be not to use \forgetparagraphfreezing globaly, but \forgetparagraphfreezing  and \setparagraphfreezing at the beginning and end of the first paragraph respectively of each chapter.  Would this be the least harmful way? That is, wrapping it up to avoid this provisional failure of the Lettrine with LMTX module, but at the same time to avoid affecting that default freezing proposal in the rest of the document, right?
 
By the way, and just out of curiosity, how does the recommended provisional command work? Would it be something like this in the preamble?:
 
\definingparagraphs
    firstparagraph][n=1]
    \setupparagraphs [firstparagraph][1][align={hanging}]
 
And then this arrangement in the \input files?:
 
\startfirstparagraph
\lettrine{B}{lah} blah, blah, blah...
\stopfirstparagraph
\blank [overlay] \strut
 
I know it's a dirty, inelegant solution, but I can't think of a better one at the moment.