Describe the bug
With my Nethermind/Nimbus combination, the sync status API always reports el_offline=true, even if all clients are in sync. This disrupts the logic implemented in sigp/lighthouse#4295, so my multi-BN setup is now essentially running only on the non-Nimbus BN.

To Reproduce
Steps to reproduce the behavior:

1. Ubuntu 22.04, Nethermind 1.18.1, Nimbus 23.5.1
2. Wait until EL and BN are synced
3. curl localhost:5052/eth/v1/node/syncing
4. Result is {"data":{"head_slot":"6504090","sync_distance":"0","is_syncing":false,"is_optimistic":false,"el_offline":true}}