Custom Power BI visual (date picker slicer) not work with SELECTEDVALUE()

One thing I'd check here is whether the visual is actually applying a single-value filter to the same model column you're using in SELECTEDVALUE().

A quick test is:

COUNTROWS(VALUES('DimDate'[Date]))

If that returns more than 1 while the date picker shows one date, the issue is probably the filter context rather than SELECTEDVALUE itself.

I'd also check that you're filtering the actual Date column and not a hierarchy or formatted text version of it.

Custom visuals can make this a little confusing because what looks like a single selection in the visual UI doesn't always mean the model is receiving exactly one value.