Na I've never seen that. It's only the second time I've tried blumats and currently I only use one while trying to get it right. The last time something went wrong by around week 6, I think P was too low perhaps (40ppm entire run) and buds were stunted. This time I am running at 60>80ppm and then lowering after week 4/5 or so. This time I also do a top feed to run off every 2 weeks with normal strength feed (it's around 750ppm), I hope this will remove any build up yet is at a ppm ratio that will not interrupt the caution exchange in coco (not that I know much about that). I will continue to run only one blumat until it is looking as good as the normal watered pots. Currently it looks ok, perhaps a little behind in stacking even though it is feeding at the same rate. Position to light and that it's from seed could account for that though.
I'd be interested if you have specifics on ppm ratios you use. Although perhaps that should be another thread if you have any insight. Also, do you get a little brown spot on the top? I can only assume it's a little bit of root rot like they get in hydro, not sure if it's a big deal in this case though. Unless I'm an idiot and it's just coco.