IDR Up Way Too High

My last loan was issued in 2011, I have been on IDR/IBR or SAVE plans the entire time, never had any issues before this past week, and the most I was ever asked to pay was ~$5 a month.

My household makes $75k a year, there have been 4 of us, and when I went to recertify my IDR plan, it went through at $0 again until May of 2026. I then had a baby, so now there are 5 of us, I'm working/earning less, AND Aidvantage has decided that I owe $400 a month and marked me delinquent without issuing any statement or notice of my repayment plan changing. AidVantage claims the IDR told them to make this change, so I went to update my application on the student aid site to include the newborn, and now I'm being told that I would owe $600 a month if I resubmit my IDR application.

Ignoring for a moment the sudden $400 monthly bill that was illegally changed without notice. How did I owe $0 a month with 4 in the household, but now I owe $600 a month with 5 in the household? This makes absolutely no sense, and I've been filling out these forms for enough years that I don't think it's user error.