PD cannot add replicas when all Stores in a zone are removed

[disksing]

Bug Report

What did you do?

Consider a 3 replica cluster. It has 6 tikv servers located among 3 zones. We use pd-ctl to delete all stores in a zone.

What did you expect to see?

As now we only have 2 available zones, PD should replace offline replicas with new peer the other zones.

What did you see instead?

Cannot replace down peer. Because PD refuses to decrease region isolation score.

What version of PD are you using (pd-server -V)?

master

[cosven]

/severity major