Maybe you do have a filed, that is set as "Required" but is not displayed on the form interface? Maybe you have a rule, that fills its value when item is added but when edited it doesn't? But it is certainly related to the required field you cannot see on your form.

For the debugging purposes you should add each mandatory field to UI and check which one is causing the problem

Regards,

Tomasz

I have found the issue, it was caused by a repeating section which seemed to be connected to a mandatory field.

Yup! Told you  Something in the form structure not certainly visible on the UI layer

Glad you have found the solution. Please, mark any from the answers as "Correct" in case you feel your problem is solved.

Regards,

Tomasz