Swap button greyed out on Azure app service slot

Clement
1
Reputation point
Up till recently I would always deploy my app to an app service slot, and then click the 'Swap' button in the azure portal to swap the slot with the actual app.
Ever since last week, the swap button is always greyed out on the slot. Why would that be?
Strangely enough, I can use the swap button on the actual app, but not from the slot.
This used to work for sure.
What are the reasons why the swap button would grayed out?