You asked about Sn 1071 (1077)
The PTS edition (p. 206) reads hitva-m-an~n~am -
-m- being seen as anaphyctic sandhi consonant, which occasionally shortens long proceeding vowel (analogously to the law of mora).
The verse is given as example of this in Oberlies' reference grammar (Pali, Berlin 2001), on p. 126 (paragr. 25).
B Scherer, PhD
Numata Professor of Buddhist Studies, McGill
Professor of Comparative Religion, CCCU, UK