This problem turns out to be a little more general.  It can be observed even if no letter options are used.

$ cat EXPERIMENTS/simple-letter.mm
.ND "17 May 2023"
.WA "Epi G. Netic" "Head of Research"
123 Main Street
Anytown, ST  10101
.WE
.IA "Rufus T. Arbogast" "Guru"
456 Elsewhere Avenue
Nirvana, PA  20406
.IE
.LT
.P
We have a research leak!
.FC
.SG
.NS
sundry careless people
.NE
$ nroff -mm EXPERIMENTS/simple-letter.mm|cat -s # groff 1.22.4

                                     123 Main Street
                                     Anytown, ST  10101
                                     17 May 2023

       Rufus T. Arbogast
       Guru
       456 Elsewhere Avenue
       Nirvana, PA  20406
       We have a research leak!

                                     Yours very truly,

                                     Epi G. Netic
                                     Head of Research

       Copy to
       sundry careless people

The problem is that there should be an unconditional `sp` request at the very end of the `let@print-head` macro, and there isn't.

Input (from DWB 3.3 mm manual, with slight modifications):

.if \nT=1 .ds Ty BL\"
.if \nT=2 .ds Ty SB\"
.if \nT=3 .ds Ty FB\"
.if \nT=4 .ds Ty SP\"
.ND "April 1, 1990"
.WA "S. P. Lename" "Manager, Product Sales"
SoftPackers, Inc.
25 Bubble Sheet Road
Columbia, Maryland  21046
.WE
.IA "J. J. Jones" "Purchasing Supervisor"
Getem Electronics, Inc.
9 Circuit Lane
St. Louis, Missouri  63101
.IE
.LO SA "Dear Mr. Jones,"
.LT \*(Ty
.P
Thank you for your interest in Softpackers' new
EnviroPack\*(Tm packaging sheets.
.P
I am sure you will find that EnviroPack offers many advantages
over other commercially available packaging materials:
.BL
.LI
EnviroPack is of a lighter-weight material,
thereby reducing shipping costs.
.LI
EnviroPack is less expensive than standard bubble sheets or
styrofoam chips.
.LI
EnviroPack is made of a completely biodegradable
compound so it is environmentally sound,
an important factor
in a world where environmental issues are of great concern.
.LE
.P
We expect this product to enjoy great success and we are
delighted that you will be among the first to try it.
The attached brochure provides additional product information.
.P
You will receive your trial shipment in one week.
If you do not receive this shipment or are not pleased
with the product,
please contact me at 201-567-9876.
.FC
.SG
.NS 3
Brochure
.NE

DWB 3.3 mm consistently puts a blank line after the salutation in all letter types where a salutation is present (all but SP).

groff mm does not.  It should.

I won't show output here as it is repetitious, but you can use

for N in 1 2 3 4; do nroff -rT$N -mm ATTIC/sample-letter-LT.mm > LT-$N.txt; done

to observe it.

(Heirloom Doctools mm behaves badly with all letter types and is not useful for comparison.  DWB screws up interpolation of the `Tm` string, and the line length for letter type BL seems to be too long, but otherwise produces plausible output.  My guess is that nroff output was not tested often or well.)