There would be a difference between you publishing your code to a public repository, under a permissive license and then allowing your company to fork the codebase and do whatever with it. Under this situation, the author retains copyright, and the company has the option to decline use of the licensed code.
That is different than solving common business problems at home, then when asked to solve them at work just copy/pasting those solutions and assuming you retain rights. Contributing that to your employer under that situation is no different than just working on salary - and you have not given the employer the option of rejecting those contributions.
See, that requires some argument of who it's for. Legally, copyright is established the moment something is created. (Hope you have proof.) I don't think you'd be able to claim _damages_ by sneaking your copyrighted code into the company repository, but other than that, I really have no idea how this would play out in court. It seems very risky but it's not obvious. I'd be interested in reading about cases in this middle area, if there have been any.
But anyway, I focused mostly on ethics. The specific situation you describe is ethically dubious, I agree, but I'm interested in where the line is and it's just not as clear as some are suggesting.
Copyright law is its own can of worms and is not the same as what's ethical. But, it does govern risk and practicality.
It's hard for me to imagine how you could lose rights via copy pasting the code. Making a new release with a new license doesn't invalidate the rights you already had.
Publishing code sounds like a way to prove it already existed, and nothing more.
That is different than solving common business problems at home, then when asked to solve them at work just copy/pasting those solutions and assuming you retain rights. Contributing that to your employer under that situation is no different than just working on salary - and you have not given the employer the option of rejecting those contributions.