Actually I just noticed - its no so much that the button locks up. It because the system clearly thinks its displaying the on screen keyboard. When the keyboard is genuinely open, that button remains highlighted, and clicking does nothing. So expected behaviour from the button, just that it isn't actually displaying the keyboard, consequently, you cannot press the close keyboard button and you are now stuck.

Thanks for the clarification.  We'll look into why the VK is not actually showing but thinks it is showing when you attach an external screen.

dvallet@ for triage.

As a side note, we're updating the behavior of this button to be a toggle as a part of implementing  issue 893856 .  So this gives us a workaround starting in M71.

OK, now I've moved on to 71.0.3578.94, stable. I've noticed that clicking the VK button now toggles, as you mentioned. 
However, surprisingly this isn't the workaround that was assumed. The behaviour is still that the VK doesn't open on external displays, as this isn't resolved yet. However, clicking the VK button again on the external display, to unhighlight the button does not seem to reset the VK display logic/problem. Therefore the VK still cannot be displayed on the main display any more. No matter what combination of clicking the VK button on external or internal display, the VK is gone for good, until you go to Accessibility menu, turn off the display of the button and back on again.

(Also, the hide VK button on VK itself seems a little redundant now toggle is in place)
cheers