When exporting the attached G3 data as an OTF, the Glyphs 3.3.1 font compiler somehow does not generate an entry for the GID+0 glyph in the hmtx table, even though an empty glyph is implicitly inserted as GID+0 by the compiler, which ends up a corrupted hmtx table. The cmap table also contains an off-by-one error that maps 0x41 to GID+0. Note that .notdef is marked as non-export, and DisableAllAutomaticBehaviour is enabled in the attached data. I got a slightly different result on Glyphs 3.4 - there are only two glyphs in the font and the first hmtx entry has 150 as LSB while the glyph slot is empty.
I tested on Glyphs 3.3.1 (3343) and Glyphs 3.4 (3414).
Such a setup might not be common but I wish it would be addressed when you have time. Thanks as always!
HMTXTest-Regular.glyphs (1.5 KB)