'H27 - Client Request Interrupted' errors when app iframe is loaded in JSM portal concurrently

We recently received ticket from a customer who reported an issue with app rendering when JSM portal was opened in 4-5 new tabs subsequently (Right click, open in new tab).

Our app is hosted on Heroku. We looked into logs and found quite some H27 errors. This is what Heroku docs says:

H27 - Client Request Interrupted

The client socket was closed either in the middle of the request or before a response could be returned. For example, the client closed their browser session before the request was able to complete.

So it looks like client browser closed the connection. But how do we know exactly why the browser would close the connection? Is there a limit on open connections (especially when connect apps are rendered inside iframe)?

Also, the customer told us this issue affects other apps as well, not only ours.