This is a known problem with none but an ugly solution. There is no "preempt" or active peer detection for ISAKMP, even though it should prefer the first peer when both become active again. If you look at the IPSEC SAs, you'll see it thinks it is sending traffic, so Dead Peer Detection doesn't help. If you clear ISAKMP sa, it will correctly elect the outside peer crypto map.
It's not IP related -- your route is correctly returning, however IKE doesn't care, it's IP connectivity is still fine on the backup, so everything breaks.
Conditionally Null route the inside address of the ASA from the outside router if the outside serial address is reachable.
You could probably do it on the ASA itself with "route dualisp 67.44.X.Y 255.255.255.255 null0 track 1".
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...