Salesforce delete Marketo fields value without any trigger action

[Györke_-_Szasz_]

I would need your help with the following issue:
I am using Marketo with Salesforce. I have found some cases (cca 80-100) when the UTM parameter fields (all 5 fields) were updated and overwritten by salesforce. It was empty in Sfdc and overwrote to null. I set it back manually and later it happened again. There’s no rule behind which should cause this sync. I set these fields Read only in sfdc, and still overwrite. I don’t want to block this field in Marketo, bc sometimes it needs to be changed
The problem is, that I can’t find the pattern. Why happened only those couple of leads? Why happened with all 5 UTM fields (and 1 more other field, but only these 6 fields), It is random, there’s no pattern when it happens (like 10 minutes after I change it, it will be changed back again). So this kind of rule.
Do you have an idea?

[Vinay_Kumar]

Well, when you update any field's permission to read-only for Marketo's sync user then Marketo can only read the field data, but cannot make any updates essentially.

If I am not wrong the data is getting overwritten in Marketo. Correct me if I am wrong here.  

This usually happens when these read-only fields get a new value in Marketo and the sync user tries to sync the new information to these fields in Salesforce, then Salesforce does not take that update however returns a NULL value which eventually updates the fields with a Null value in Marketo. 

[Györke_-_Szasz_]

Thank you for your message
It wasn't read-only originally, I just tried it as a solution., but I set back to 'read and edit'
I don't understand honestly how I should solve this problem. Why do I have this issue in some cases, randomly? And why only in some cases? Here you have a print screen from an activity. There are old leads, new leads, no pattern either