Update: Issue seems to be resolved
Immediate VPN connection with the Android 10 r5 and the current android 9 release when using the phones existing VPN settings (i.e. I changed nothing apart from installing the Beta version)
Looks like something has changes between releases as I no longer get the following errors logged:
msg: packet shorter than isakmp header size (0, 64, 28)
&
msg: phase1 negotiation failed due to time up.