Users losing secondary smtp from ProxyAddresses in a two way sync

Very strange one here.

We have had 2 users over the past few weeks now lose their secondary ProxyAddresses, which seems to tie in around the same time as a password reset.

So for example a user has:

SMTP:user.name@domain.com
smtp:user@domain.com
sip:user.name@domain.com

then they just have:

SMTP:user.name@domain.com
sip:user.name@domain.com

We've had the lower case smtp disappear for two users now, and the one I am reviewing at the moment lost two secondary smtp addresses.

The logs are not very clear, just wondering if anyone has any ideas?

This user is NOT in the scope for the two way sync, they are in an OU that is only syncing from Source > Destination.

We are using Azure AD Connect to sync up to O365 also.

Any ideas/thoughts?