The primitive form of this word in later writings is unclear. The development in Quenya seems indicates a primitive initial ✶mb- and a primitive form of ✶mbeleke, as in ✶Mbelekōro or ✶Mbelekōre > Q. Melkor (PE17/115, WJ/402). However, the nasal mutation meleg appears in Taur-i-Melegyrn “Forest of the Great Trees”, indicating a primitive initial ✶b- and a primitive form of ✶beleke. It is possible the soft mutation veleg appears Arveleg, also indicating a primitive initial ✶b-. @@@ Also Cûl Veleg.
It may be that the strengthening of √BEL ⇒ √MBEL occurred only in Quenya, and the Sindarin form developed from the original √BEL instead. Since The Etymologies state that √BEL was “not found in Q[uenya]”, this seems a likely scenario to me.