As far as I remember, during the BC provincial elections in the fall, Mainstreet consistently indicated the BC United (conservative) party was around 5 points ahead of the NDP in polling.
The others generally showed it to be a dead heat.
In the end, the NDP won a very close race and Mainstreet was shown to be the one overrepresenting conservative vote intentions as compared to the other pollsters.
Not sure what the differences were in their methodology, but it wouldn't surprise me to see the same thing going on here.
I would say the reason the NDP, the Bloc, and the Green vote has been collapsing is that this is an election where the key ballot question is one of foreign policy, particularly to do with the US. The NDP, the Bloc, and the Greens are all primarily domestic policy parties, so they're not speaking to the ballot question. The Liberals have a leader who does speak to the ballot question, and so do the CPC in that he's largely aligned with the IDU and MAGA. So, it's a two-party race this time. Doesn't mean it will be that way next go around.