I think it’s that often signs are produced months ahead of time, and these signs must have been produced at the moment they did the shift or just before.
On top of that, it could also be something with the printing shop not yet adapted/equipped to printing the new signs with numbers just yet.
The last reason, the one I believe the most probable, is that the STM does not want to mix signs with just the dot, and signs with a number inside the dot. This is because the signs along the quai perhaps are not ready yet to switch over to the new system with number + dot and perhaps other types of signs. They have currently done one station sign at Henri-Bourassa, and I bet you they are just testing to see how the new format holds up, before rolling out the dot+number to the rest of the network. I think they are still updating their existing sign design, updating the maps, and the next signage change that they could do, and all the different signage tools have been transferred over to the new format, then they would add the dot+number.
They just don’t want stations with many different era’s of signage.
EDIT: I notice that the signs are actually 2022’s version of the signs. It uses the wide arrow, versus the newer 2023+ design uses a thinner stroke arrow for dark background signs, as seen at Radisson:
Currently only Montmorency & Radisson have this newer signage format.