Visual editor blank lines behavour in multi-site root

Hi, this isn’t a WPMUDEV support question as such (I don’t think)

On my sub sites teh visual editor behaves well, in that if you put in blank lines it inserts   however on my multi-site root site it doesn’t so any blank lines are effectively stripped. (I’m assuming it isn’t related to WPMUDEV plugins, but I’ll go away and test that possibility)

Any one else seen / solved this?