Lightsail Instances "Loses" IPv6 Address

Hi,

I've been running a AWS Lightsail instance for a few years now. I read that IPv6 was now default for AWS Lightsail instances, and can be activated for older instances like mine. I've the steps to enable, but the error mentioned in instructions isn't resolved.

Immediately after booting, running ip addr shows my IPv6 address as specified by AWS, but with the suffix as follows:

inet6 [address] scope global dynamic noprefixroute valid_Ift 429sec preferred_Ift 119sec

Sure enough, if I run again shortly thereafter I see:

inet6 [address] scope global deprecated dynamic noprefixroute valid_Ift 308sec preferred_Ift 0sec

and if I try again a few minutes later, the IPv6 address is gone.

Any idea what could be wrong here? I'm very inexperienced with linux and networking in general, so I've tried to keep to fully standard processes with AWS, and haven't customized much of anything.

Thanks for reading.