How to recognise electro magnetic mode in patch antenna based on current voltage profile
The H field current is strongest just under and around the feed and the asymmetric geometry of the feed point and ground plane rectangular shape affect the H field on A/m and consequent return loss vs Theta.

https://www.hindawi.com/journals/ijap/2017/5924157/fig3/
With free space impedance and ground plane impedance to patch with some gap, the edges aren't exactly open circuit, yet a partial strip ground patch is adequate to conduct the current for the patch to radiate an EH field.