Strangely, if I click in the reply box on this forum my keyboard pops up then disappears, it won't stay there to let me type. But if I select it from settings it stays ok and let's me type.
If you use the Enhanced Interface WYSIWYG keyboard on the forum, then it stays open. The Basic and Standard seem to glitch and close. You can change your default under Settings - General Settings - Message Editor Interface. Hope this helps with the disappearing keyboard on the forum.