I‘m having the exact same issue on my iPad Pro 11“ since ages. I think it persists since the first iPad OS with external monitor support. It first occured to me with a 3rd party bluetooth keyboard. First I thought it might be some bluetooth issue, so I bought a Magic Keyboard. But the problem is still there on a daily basis.
My setup:
- iPad Pro 11“ with M1 Chip
- HP Z38c Monitor connected via USB-C
Some keys work all the time, like for example Cmd + Space to invoke Spotlight search. But as soon as there is an text-input, it‘s game-over.
Sometimes I‘m having limited success by force-quitting apps. Then text input works for some time. But sometimes nothing helps except restarting a device.
I think I will sell the iPad because of this. Currently it‘s my main computer for private usage. It‘s a pity because if it just worked, the iPad would offer everything I need besides work.
But the issue is so annoying that I will switch back to a Mac or maybe a Microsoft Surface.
I‘m also really dissapointed to read here, that the issue is still there on the newest M4 models. Such a pity: beautiful, powerful hardware, but a buggy OS that‘s lettings users down :(