What was happening is that apparently on Odoo you can't write a value to a readonly field, so on my XML I doubled my field that was readonly, and the new one I made it just invisible, and now it works perfectly. This is something that I also discovered, you don't need to declare a new field on the .py file and create it on the XML, you can just copy and paste your field, remove the readonly attribute, and add an invisible attribute, works perfectly.

https://stackoverflow.com/questions/50845078/odoo-readonly-field-doesnt-save-value-on-onchange