Are you getting this problem reliably? Do you have it with other documents? Would you mind sending me the document so I can look at it?
The only thing I can think of off the top of my head is that this is a monster document and PS is lagging the changes due to resources. But I'll be honest, the way switching to bold and italic are working for you, I suspect there's some bug in there -- hence why I'd like to play with your doc.