I’m quite aware that the IPA is already contaminated and contains dissolved resin, but that’s not what I’m talking about,
I’m talking about all the undissolved, resin particles that end up at the bottom of the tank like sediment. You put an impeller at the bottom of the tank, and you promptly swirled all that sediment back into the liquid, leaving it free to deposit itself all over you “newly cleaned” part.
I can’t speak for anyone here, but I know that in my case, I can fill a bucket with fresh IPA, and within 8-10 prints there’s enough crud at the bottom of the bucket that I need to filter it out. I know that filtering it doesn’t actually remove the dissolved resin from the IPA, but it cleans out the solid particles. That’s why I have the “clean bucket” to remove the film of contaminated IPA.