Live captions can only be one line long. Convert them to static captions. Object > Captions > Convert to Static Captions.
I was trying to avoid converting them to static captions for obvious reason, but if this is the only option then I guess il have to run with it.
Yeh this is another not very thought out concept from InDesign.
Well it's based on the Text Variables feature, and it basically uses a Text Variable to create the text pulling the information from the metadata.
You get the same issues with Running Heads and using a Text Variable for this, the running heads get all squashed up if you haven't left enough room.
http://in-tools.com/products/plugins/power-headers/ - this fixes the running headers issues.
But I'm not sure if it will work for captions?
And to answer your question.
This is how I overcome the issue.
I set up the Live Caption on one layer.
Once they are all generated I see them all squished up.
I copy them all to a new layer - and then you can convert them to Static Captions.
Turn off the layer for Live Captions.
Of course now - you have to be really careful and generate Static Caption layers anytime an image on a page is generated.
Unfortunately I don't ever do running heads so that would be an expensive plugin should it not work. Although I may contact them to test it for me and purchase if it works.
There are around 20 charts across a 40 page document so its not too drastic should they start swapping things about, just an inconvenience and waste of time.
Looks like static captions are the way.