troffcvt with -mm option gets all font sizes wrong

[LukeKendall]

If you use the -mm option on troffcvt, it always outputs wrong \point-size definitions: most of them seem to end up computed as 1. (I think, point size N gets gets turned into point size 1 if N < 12; otherwise gets turned into point size N - 10.)
I suspect that it's the .H and .HU macros (for creating headings) - this simple input file works correctly:
.ps 14
.vs 16
.ce
.ft B
troffcvt\-a troff converter
.ft

This one doesn't:

.ps 14
.vs 16
.H 1 "Heading"
.ce
.ft B
troffcvt\-a troff converter
.ft

Here's a slightly longer example:
.ps 10
.vs 13
.HU "Heading"
.P
Now let's try the following word 4 point sizes smaller: \s-4SMALL\s0.
How was that? And now one 4 points larger: \s+4larger\s0. How was that?