Google App Engine: manual scaling with one instance suddenly created multiple instances

Question

I have a Google App Engine application with

runtime: java17
instance_class: B4
[...]
manual_scaling:
  instances: 1

It has been running as set up with 1 instance for years. Suddenly this night (2024-03-26 around 3:15 UTC), without me doing any change, it scaled up to 4 instances, causing various issues. How is this even possible, and is there anything I can do to stop it?

Answer 1

Update: all good now, after 38 hours, the service ramped down to one instance again. Meanwhile I tried to restart and re-deploy the service but nothing helped. Turns out it was indeed an issue at Google's side - see the post I made over at Google's forums for more details.