It looks like an update went out this week to Azure DevOps boards.
I used to be able to control-click to select multiple items (like bugs) and use the ::: and choose Edit and then select a field to update on multiple items.
In the list of fields to update, I had a field called "Found In" which is a single line of text.
Before this week, this field was available as one I could update in bulk, but now it is gone - why would it no longer be available as an field I can update? I can update it by opening each bug individually.