An upright or poorly angled item can retain wash water rather than allowing it to run away. The model's rack supports are intended to help position suitable cups appropriately.
Understanding this situation
A recess in the base of an inverted mug can still collect a small pool, which differs from the entire load failing to dry. The shape and placement need to be considered.
A useful next step
Load as directed and unload carefully to avoid spilling retained water onto dry dishes below. If all items remain wet or a warning occurs, report that broader result rather than diagnosing the dryer function from one mug's recess.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.