The incident was caused by multiple ingesters being unavailable at the same time due to moving ingester pods between nodes. It's a regular operation, but in this particular case the ingester took an unexpected long time to restart which coincided with another ingester eventually restarting at the same time, causing an issue.
Spain public probe was facing issues and had intermittent failures between June 21st 20:00 UTC to June 22nd 8:40 UTC. Synthetic monitoring checks using Spain public probe have been failing intermittently during that window. The issue is now resolved - the probe doesn't suffer any issues anymore and is stable.