I am trying to use the WHERE parameter in the URL of my experience view
I wonder if this is related to problems with the underlying search mechanism used across the platform. There are a few posts about various search issues in the forums, couple linked below:
It seems the whole 'contains' type search doesn't work in a way that end users would expect. Not sure if this helps but it might be worth logging an Idea on here if you can't get it to work or maybe log a support case with your local Esri support team.