I was having same problem with safari iOS 15 and IPad. Keyboard would not pop up when clicking into a text entry field. Rebooting the IPad corrected the issue for me
Appreciate the thought, but it only ‘loses’ built in keyboard on pages saved as shortcuts on the desktop, no problem if I just open safari and go straight to it.
I’ve been having this issue for at least a couple months on iPhone 12 pro running iOS 15, 15.1, 15.2, and now 15.3 beta. If I force close the shortcut it works properly when I reopen, but I have to do this every time or the blank keyboard pops up.