io_tester throws std::bad_optional_access intermittently

[robertbindar]

In some runs io_tester outputs many lines of:

ERROR 2025-10-10 12:52:40,736 [shard 46:main] seastar - Timer callback failed: std::bad_optional_access (bad optional access) 

Nothing special about my config:

- name: big_read
  shards: all
  type: randread
  data_size: 10GB
  shard_info:
    parallelism: 12
    reqsize: 512