Hi Zahrah,
Thanks for the information. I am wondering if the web app working properly to prevent blank required submission?
For the feature layer on ArcGIS online, the required question has the setting nullable:"true" by-design, this is because sometimes the required question is used as a relevant question, and when it is not relevant, the required question should be hidden and allowed to submit as null.
Could you please let us know your use case for having the feature layer field as nullable:"false", so we can log your requirement as a feature requirement. I also encourage you to contact Esri Support and raise an Enhancement issue.
Thanks,
Ruth