Safe L2 head doesn't advance

When running the default op-stack recipe, the sequencer does not progress as it should, as indicated by this log message in op-batcher:
```
lvl=warn msg="sequencer did not make expected progress" ..... syncStatus.safeL2=8550c1..b0e0a3:0 ....
```
This message is logged regularly with no change in `syncStatus.safeL2`