Here's a simple MWE:

Default {\definedfont[ALMFixed]ALM} Default

I have the "ALM Fixed" font installed (otf from https://ctan.org/tex-archive/fonts/almfixed).  I do not see any errors in the output.  In particular, Context sees the font and the logs contain the following line:

mkiv lua stats  > loaded fonts: 3 files: almfixed.otf, latinmodern-math.otf, lmroman12-regular.otf

Yet, the output from lmtx does not display the "ALM" string, though its place is reserved on the line.  The same file typesets fine with mkiv.