They have super high value cargo sitting at the remote port wanting to be moved. Loading empties takes time and pays little, so they profit optimize by not waiting for them to be loaded, and instead abandon them to clog the port so they can get out a few hours earlier and do their next run faster.

Is it profit-maximizing when they have to wait a week to unload when they come back with the same ship or a different ship?

If not, you think that they would eventually work it out

... and get clogged themselves next time they come in?

It could be a tragedy of the commons effect.

If one company alone were to max out on empties, the port would still be clogged.

It takes time to load them, and shipping rates from Asia to the US are at an all time high, so they want to hurry back.

Is it speculation or fact that the ships are heading back empty?

Not empty, but significantly below what they could carry.