Why do I owe money from having Obamacare for five months of the year and then switched to employer healthcare for the rest?

[MichaelMc]

If your were receiving an Advance Premium Tax Credit as a monthly subsidy to your ACA health insurance coverage, that subsidy was based on your then-current income as you would have reported it when you purchased the insurance.

I suspect that along with making benefits available, your employer was paying you enough of a salary that it increased your monthly income above your original estimate, and you now have to repay some or all of the Advance Premium Tax Credit.