To add to Rick Joe's post & confirm Terry Coker's comment, it seems it works only with "plain paper" setting. Any other paper type choice, no go.
Yet ADF worked for scanning whatever paper type was set.
In addition on my ET-3750, re copying, the ADF indicator icon always came on when you inserted a page (or pages) in the ADF, regardless of the paper setting, so that made it weirder than it was. Seems like the alert message would more usefully advise of this, instead of "use scanner glass".