Yup. I’ve had same issues using one of these.
If I plug the iron in, it’ll usually negotiate 12v straight away, but shuts down after a minute or so. If I’ve turned the power bank on, it’ll refuse to negotiate 12v. I wasn’t sure if it’s a power bank or soldering iron issue, sounds like it’s the Pinecil though.
I’ve got DC barrel jacks and an adapter to use my drill batteries, I just need to finish putting it all together, should solve the problem.