I'm pretty sure that the flashback is supposed to end at the point where Mitsuri calls Yushiro's maneuver on Nakime cool. In the panel after that she isn't talking with Yushiro anymore, has her sword out and is mid air, so that's clearly not the same point in time anymore. Furthermore, it would be strange for the flashback to jump like that without an indication since we haven't even seen Yushiro approach Nakime to hijack her at that point. The page after Mitsuri's statement about the castle moving also shows her continuing her previous thoughts while also showing Muzan, so that's clearly supposed to be the present already. Also, the borders of the page with Mitsuri's statement regarding the castle transition from black at the top to white at the bottom which is probably supposed to indicate that the upper part is the flashback and the lower part the present. It's probably because of an incompatibility with the website, try copying the URL to another tab. I would personally not be so fast to assume an art error and I would consider arguments that rely on that sort of assumption suspect by default if there isn't any further evidence but I do concede that authors do not necessarily keep track of that on a precise level though I do think that Nakime being able to vary the size of her pillars makes perfect sense with the way her ability is portrayed and described. Not really, I only realized the size differences with the pillars today, so I couldn't have brought that up with Muichiro. The things that we discussed in regard to the pillar that hit Muichiro are Gyomei and Muichiro's reactions to that happening, Muichiro's state after crashing through a wall to the place where he encounters Kokushibo and how this happens while Nakime is fighting Mitsuri and Obanai.