ILM unable to delete old index since upgrade to 8.19.20

Morning Team,

Since upgrading to 8.19.20, I've started getting these errors:

policy [.fleet-actions-results-ilm-policy] for index [.ds-.fleet-actions-results-2026.05.02-000014] on an error step due to a transient error, moving back to the failed step [wait-for-shard-history-leases] for execution. retry attempt [143]

policy [.fleet-actions-results-ilm-policy] for index [.ds-.fleet-actions-results-2026.05.02-000014] failed on step [{"phase":"delete","action":"delete","name":"wait-for-shard-history-leases"}]. Moving to ERROR step

Problem just concerns this one index.

I've tried deleting manually but no joy.

My instinct is to leave things alone until [at least] after our monthly upgrade, next week.

I'd appreciate any suggestions as to cause of the problem, best way to 'fix' it, and how to prevent it happening again.

Many thanks.