Something about setting multiple Source Models of a UX Page
I noticed the following message when setting the source model of a UX page.
"Only models that you have access to are displayed. 1 additional model is associated with this page"
I doubted the reason and finally realized what happened. One of the associated model was archived.
So even I removed other models and left one model on the page, the [Enable multiple source models] toggle did not become active as there is another archived model still associated with the page.