Each council chooses their method of posting voting papers. Either RM or another company which I've not heard of before, Adare SEC, which diverts mail through the Netherlands regardless of where it's destination is.
The government and some councils have not put clear info on their websites to let expats and EU nationals know the hows and whens of applying for the forms needed to vote in any one country and, more importantly, the date by which they had to be returned (May 7th) to the issuing office.
Adere SEC say that all mail was delivered within the approved timescale.
The Electoral Commission had nothing to do with it. Local councils each made their own arrangements.