In Donatus we have the same fact stated, with the same reason:

[Keil. v. IV. p. 442.] Quod magis aurium indicio quam artis ratione
colligimus.

Thus we find numeral abverbs and others ending either in _iens_ or
_ies_, as _centiens_ or _centies_, _decies_ or _deciens_, _millies_ or
_milliens_, _quotiens_ or _quoties_, _totiens_ or _toties_. Other words,
in like manner, participles and nouns, are written either with or
without the N before S, as _contunsum_ or _contusum_, _obtunsus_ or
_obtusus_, _thesaurus_ or _thensaurus_ (the _ens_ is regularly
represented in Greek by [Greek transliteration: aes]); _infans_ or
_infas_, _frons_ or _fros_. In late Latin the N was frequently dropped
in participle endings. Donatus says that this nasal sound of N should be
strenuously observed:

[Keil. v. IV. p. 442.] Illud vehementissime observare debemus, ut _con_
et _in_ quotiensque post se habent S vel F litteram, videamus
quemadmodum pronuntientur. Plerumque enim non observantes in
barbarismos incurrimus.

GN in the terminations _gnus_, _gna_, _gnum_, has, according to
Priscian, the power to lengthen the penultimate vowel.

[Prisc. I.] _Gnus_ quoque, vel _gna_, vel _gnum_, terminantia, longam
habent vocalem penultimam; ut a _regno_, _regnum_; a _sto_, _stagnum_;
a _bene_, _benignus_; a _male_, _malignus_; ab _abiete_, _abiegnus_;
_privignus_; _Pelignus_.

(Perhaps the liquid sound, as in canon.)