It is known that DMS partially decomposes in aqueous solutions. How about TMP? Adding a little bit of acetone to the DMS reaction mixture boosts yields and keeps the amount of DMS needed (mol eq wise) low.
Post 507716 (https://www.thevespiary.org/talk/index.php?topic=9924.msg50771600#msg50771600)
(GC_MS: "Permethylation of pyrogallol with DMS", Methods Discourse)