The assembler it was unloading was backed up:

The inserter would not unload:

The chest is not full, the assembler shows "item productrion overload" and the inserter shows "waiting for source items".
I would expect the inserter to unload the assembler in this scenario.
Log file attached.
Save file attached.
The only mod is squeakthrough.
Edit: This appears to just be a misunderstanding on my part. The chest is actually full- offshore pumps only stack to 20, which I didn't realize.
That being said, it seems like the inserter should display a more appropriate status message than "waiting for source items" when it is in fact waiting for space to offload.