Paper UI / Basci UI ... cross lock each other?

when I open one of these UIs the other won’t open until I close the tab of the first opened…

is that normal?

From a PC or from an iOS device?

PC, Win 10, Chrome Browser

once I close a tab (eg paper ui) the basic ui instantly loads and vice versa

I would assume that this is a restriction of the OS to not allow more than a certain number of HTTP connections to the same host at a time. I remember that iOS had a limit of 2 in the past (at the time I developed the Classic UI).
But by now, 6 seems to be the minimum for most browsers:
I do not hope that either of the UIs requires more than 2-3 simultaneous connections, so I cannot really tell you what might be the issue here.
And no: On Mac with Chrome, I have never seen this and I can have both UIs open.