Yes, from the very few characters we actually looked and had memory loss, we know they didn't change drastically. They never really acted the same, because Tahu was much different after his memory got swiped than before, because before he had learned how to be a team leader, while after he had trouble with working together.
TBH, there's no established rule about this kind of thing in the Mataverse. The majority didn't change, but that doesn't say there is no possibility or room for exceptions. It's your own interpretation, but it's not a rule that says that amnesia will never lead to character changes.
So who knows ? Perhaps Pohatu will change to become more like his older character over time ? Perhaps not ? Perhaps he'll be completely different. We've seen character changes in the past, some even quite drastic ( Kongu from being militaristic serious to more lay-backed, etc. ), it's not really unlikely it won't happen in the future.
This is something we'll have to let go in the future, because this "everything needs to be 100% solid and canon" attitude doesn't seem to translate to the new Bionicle, at least not from what I've seen so far. To me it looks like things are a lot looser than they used to be, and I'm really glad for that.