I don't use an iPhone, but have seen similar complaints on Twitter from folks who upgraded to iOS 13 and not only does the Echo Auto not work, they can't even connect to their car via Bluetooth any longer.
So I think you're right that the upgrade to iOS 13 is the culprit.
Today I got the same blump noise response, but it was working yesterday after I upgraded to IOS13. I’ve tried both aux and Bluetooth in communicating with Uconnect on my jeep. I just get blump after setup.
Update - my echo auto now appears to be working again. I didn't really do anything, so maybe Amazon updated or maybe I'm just lucky or maybe IOS13 decided to give me a break.
I am having the same issue. Uninstalled and reinstalled the alexa app to allow access to bluetooth and nothing has working since upgrading to ios 13. It is hearing me prompt and I am getting the same bebump as others after saying alexa but it doesn't respond to any commands.
I just got my Echo Auto after I upgraded to IOS 13. Connected Auto and it connects fine but I get no sound from car speakers and auto does not respond to questions. When I say Alexa, blue light comes on and I get the sound that says it is listening but no response. Then beep again and goes off. So don’t know if issue is related to IOS 13 and if it would have worked before IOS 13.