When using a single select slicer and publishing to the web it, it does not snap to the first value in the list but keeps the default, even if the default no longer exists.
The power bi desktop forces you to select a default value, which when published to the web and having one report serving many customers with RLS may not be relevent. Therefore reports appear blank until a new value is selected, whereby the default disappears from the list.
We need the single select slicer to be able to clear the default selection and snap to the first in the list, this used to be the behaviour and it must have changed recently. Until this happens the single select slicer is not useable for enterprise solutions with many customers and RLS.