Lines grouping together even though they have returns causing gaps

I'm having to deal with a problem that I haven't come across in 20 years of using InDesign.
A client has supplied a file where, for some reason, lines randomly seem to want to group together which creates white spaces mid-sentence at the bottom of a paragraph.
I've tried various workarounds, such as rewriting the passage and pasting it in, but nothing seems to get around it.
Has anyone else encountered something like this and found a workaround?
I'm suspecting the problem may be in the clients original file which is a 120k word book.
I've attached a couple of screenshots to try and help explain what I'm talking about.