Help! Backdoor got screwed up by Fidelity's wrong advice

Hi, 

I did backdoor IRA in early 2023. Because I'm no longer in my previous company, Fidelity usually called me to close my previous company 401K account to transfer to my personal accounts (traditional IRA).

They guaranteed no tax events will happen. However, my CPA just told me because of the transfer, my traditional IRA's balanced increased from $0 on 1/1/2023 to $100k on 12/31/2023, made my backdoor IRA contribution invalid, and I have to pay IRA tax!

Anyone knows what I can do, or push Fidelity does to fix this issue? 

Thanks a lot!