yes, read-only doesn't make sense as a solution in this case. I would suggest two things to start with here.
1 - take a look in the activity log of a handful of leads where the lead score is being wiped to 0. You might be able to see if something changed and is actually happening on the Marketo side that is causing these scores to reset.
2 - if you confirm that Marketo is in fact not the culprit, check with your company's Salesforce Admin. It's possible a new SFDC workflow (equavalent to a trigger campaign in Marketo) was created on the Salesforce back-end that is erasing the lead score assigned to a lead by Marketo.
Lastly, if the lead score field has actually been made read-only on the SFDC side that could also explain this, because Marketo wouldn't be able to write anything to that field in the first place. Hope this helps as ways to start investingating