WordPress 5 cursor keeps jumping to random places

When I’m editing text in the new WordPress 5 the cursor keeps jumping to the beginning of the paragraph, or to another paragraph. It’s infuriating and causes typos if I don’t spot it has jumped.

It’s also difficult to apply styles to text. For example, if I highlight a word with the intention of making it italic, the word becomnes unhighlighted when the cursor jumps and the whole paragraph is made italic. Same thing if I highlight a word with the intention of typing a new word to replace it.

It all makes editing text fraught and time consuming.

Is this a known bug, or is it just my setup.

I’m using the ShopIsle theme.