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.
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