I don't think anyone fully understands how Frontier vouchers and credits work, but if you have a service interruption, I would use the chat or phone call feature to make sure you get a voucher that is good for all ALL costs of a new ticket -- not just the (usually modest) base fare. They can do this. I know because they've now done it for me a couple of times, after previously giving me a voucher that was only saving me like a $1.40 on my next ticket (with a short voucher expiration date no less!).