We did manage to get this resolved with the help of our friendly neighbourhood Marketo support -- here's what the issue was:
Sounds like this is a formula/calculated field. If that's the case, this is the reason why Marketo didn't detect the change. You can add a SFDC boolean custom field to the Account Object such that every time "Number of Opportunity Archive Pause" updates, it will also update this boolean field (if the boolean field is false => set it to true. If the boolean field is true => set it to false). If you create this new account boolean field in SFDC and flip it to true initially, it will force the account object to resync to marketo.
And a related community discussion that talks about this:
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PuAcAAK