As far as I am aware this issue would be coming from the theme, but I'm not familiar with the theme's code.
I'd be happy to have a look at it for you if you like, but I would have to request access to your store so that I can view the theme files.
Let me know if you would like me to and if you do I will just need to grab your store URL (you can send it privately to me if you like).
This is really strange issue, as per your description it sounds like that theme code use filter to ignore <br> tag.
Please share your store url or if you want you can shoot me a direct email and I'll check it for you.
|a minute ago|