I think it's worth acknowledging over your customers that the bug has caused them lost time or other kinds of issues. Whether you do that with an apology, it doesn't really matter, but I know I'd personally feel better when a vendor emphasizes with me.

Something you definitely don't want is to sound like it's an inconvenience for you that they're reporting bugs. Bug reports are extremely helpful and you should be grateful that they're taking the time to report them to you.