There's been an external keyboard bug in Crosswords for as long as I remember. I use the Smart Keyboard, but I imagine it's an issue with every iPad keyboard.

Open Crosswords with the external keyboard attached. The Crosswords software keyboard is automatically hidden. This means that certain functionality, like clearing all errors (which normally you can find by long pressing on backspace software button) is inaccessible.

There is a workaround to this issue, but this workaround reveals a bug. If you disconnect the hardware keyboard, it brings up a software keyboard. But the software keyboard it brings up is not the Crosswords-specific keyboard – it's the standard iPad OS software keyboard which does not have the extra functionality like "clear errors". But you can hit the 'hide keyboard' button on the standard iPad OS software keyboard, and then tap again on a clue, and it finally brings up the Crosswords-specific software keyboard where it reveals this functionality.

The "nicest" solution would be if the special function buttons like "clear errors" were available on the bar at the bottom when a hardware keyboard is attached (perhaps in a menu button or even in the menubar that comes up when you hold the command key or both). The next best solution would be for the Crossword-specific keyboard to show up when disconnecting a hardware keyboard instead of the standard iPad OS one.