Just my non-native take on it:
Yes it’s similar in South Wales. It’s difficult to be certain of how to pronounce some place names, depending on which language you are speaking and not wishing to come across as pedantic or on the other hand, lazy. Ive been corrected both ways in the past, but mostly in a helpful way.
The wy sound can be tricky: oo-i or oy. Also Ll , ae, au, eu, etc. There’s no infallible geographical pattern to it - I find it easier to just copy someone. Or a couple of people, one speaking Welsh and one English
Anyway, if it gets weird, I keep this in my back pocket - Not many people pronounce my town, Newcastle properly (not even me).
A question to finish, please:
What is the correct “whys” pronunciation in Gwenwhyseg, please?.