The velar nasal generally become [g] initially (PE19/19), while medially it usually became [ɣ] when then vanished (PE19/23). It survived only in the cluster [ŋg], both from original primitive [ŋg] and also as developed from medial [ŋk] and from [ŋw].
It seems that the combination [ŋg] could also appear initially in the Doriathrin dialect, where it might have reduced to a simple nasal [ŋ], allowing this phoneme to appear again as an isolated sound.
Reference ✧ PE19/23 ✧ -ŋg-
|ᴹ✶-ŋw- > -ŋg-||[-ŋw-] > [-ŋgw-] > [-ŋg-]||✧ PE19/23|