I am getting the same thing about 1/2 of the time when I use appex.get_url() to get a shared url from Safari. To test this I am using the built in demo “URL to QR Code. When it works, the QR code appears quickly. When it does not, then the extension does nothing for about 2 seconds and then shows “No input URL found.” Both of these will happen on the same page, and it seems that once it finds no input the page will not work again until I force close Safari. Sometimes it finds no input on the first run and sometimes it take two or three runs. This does not happen when using other browsers.
Context Notes:
Pythonista Version - 3.2
iOS - 11.2.6
iphone 8 plus