I have recently discovered that having Attribute Selection enabled in an Experience Builder (both Enterprise and AGOL) effectively disables the ability to hold the SHIFT key or CTRL key to add to or remove from an active selection. I am wondering if this is by design (a poor design in my opinion especially since it does not tell you that ability will be disabled) or if it is a potential bug?
https://developers.arcgis.com/experience-builder/guide/select-widget/
It is a strange limitation, but it is documented. Note, that if you load a layer at run-time either through the Add Data or a Custom Widget, you can actually get around this limitation.