There's a distinct possibility that the problem is not so much the slipped gasket, as it is that the slip gasket caused some other problem that caused the membrane to fail internally. While I've had few problems myself, I know that RO membranes have a reputation for being fiddly and delicate. Putting the seal back where it's supposed to be seems to have restored normal operation (as far as I can tell, meaning roughly 100GPD permeate output) but I'm ordering a new membrane anyway. And it's almost filter change interval so we'll just do the whole thing. This membrane is only 8 months old and shouldn't need replacement... but they only cost $35, so not worth fiddling with it too much considering that the high PM2.5 seems to have been causing my furnace filters to clog and I have totally spent more than $35 on filters while troubleshooting this.