Requested a Payment Plan, never got a response

Maybe I'm just dense, I'm not sure...

When filing my taxes in February, I was going to owe some in Federal. I decided to choose the option to request a payment plan. Went through all the steps, submitted my filing and it was approved within 20 minutes.

When I got a copy of my return from the Turbo Tax website, it had all of the payment vouchers included in the pdf. This is the part where I must just be dense because I took this to mean that the payment plan had been accepted, and I would just make my payments according to the vouchers.

Come April, I made the first payment (1 of 4). Made another one a couple months later and never heard anything. 

Yesterday, I got a letter in the mail from the IRS saying I was negligent because I had not paid my balance in full - hence this conversation.

Was I supposed to receive a notification from Turbo Tax about the acceptance of my payment plan? Where in the process did I mess up? I haven't deleted a single Turbo Tax email in over 7 years, and I haven't changed mailing address in 3 - did they just not send me something, or did my mail carrier mess me up?

I have since contacted the IRS, they confirmed that I was not on a plan, and I set one up through them - but I've now got additional fees due to the whole process being wonky and so wildly unavoidable that it's irked me quite a bit. I put a little too much faith in Turbo Tax facilitating the payment plan request and got thoroughly burned, lesson learned..