I got the impression that though there is a tendency for “a” to be used before a consonantal form, it is by no means a cut and dried thing.
Certainly what I got from the conversation with the above people about it’s natural use was, as I said (easier to cut and paste!) “Though in English it is pretty cut and dried- you use “a” before a consonantal “i” or “w” sound, and “an” before a vowel “i” or “w” sound [a young man, an evil hamster; a well trodden path, an oozing sore], in Welsh whether you use “ac” or “a” before the consonantal forms tends not to be so cut and dried- at least in speech. Though officially it may tend to “the other way round” than in English, it seems pretty much left up to the individual. 
”
Just my impression of the impression of the people I mentioned above though, of course!
I’m not sure I would describe using “ac” before what might be regarded as a consonantal “i” (or “w”) as wrong and a hyper correction (no quotation marks! ) - Just another person’s natural use of the language.
[Correcting “ac” to “a” or vice versa in either situation unasked would, of course, both be hyper corrections!]
Edit- in other words, “use ‘a’ before the sound represented by “i” whenever it has a consonantal value” is not a “rule” you will find many places, because (unlike in English) it isn’t universal practice.
Edit2 - I was going by my experience and the experience of others I had talked to. But stuff like the “Gwasanaethau Mamolaeth a Iechyd Plant” mentioned above is of course relevant. For fun, if you google “ac Iechyd” and “a Iechyd”, you will see many, many sources using both forms.