Backspace/Del doesn't delete text in text module in Upfront

There are text modules with some text (grouped) on this page Edit one of them, select a block of text (preferably multi-line) and hit backspace or delete. In most cases instead of actually deleting that selected block, it only removes a small part of it (e.g. two letters of the first word), un-selects it and moves to cursor to the starting point of the selection.

That happens also when you edit posts via Upfront and also on other sub-sites of that multisite (however only this one is the one that can be “freely tested”, other’s are “production” sites).