Steps for reproduction:
1) Create a new scene
2) Create a lightmap static cube and a plane
3) Delete all the lights
4) Create a new Area Light
5) Observe that instances don't cast shadow when light source is an Area Light

Notes:
- A workaround is possible, by switching Type of another light type (i.e. a Directional light) to Area light
- Screenshot is attached below
- A reproducible project is also attached below (can be used as an alternative to reproduction steps)