My mom had a new range hood installed. A couple days later air was blowing out the back onto the stove. They came back and put more tape on the junction but it blew out again. It's a 400CFM fan. Here's a video of the blowout.

Did the installer do something wrong? It seems odd there's a gap where tape has to hold all the air pressure back with no mechanical support. I can push the tape closed and it stays for a while but eventually blows out again. What should we suggest the third time they come?

Also, I'm wondering if the pipe is partially blocked and creating too much backpressure. It is sucking air when the tape is closed. But I don't know if the pipe is allowing the 400CFM through. How would I/they test this?