I just upgraded to WP 5 and also experiencing this problem. When I switch to the code editor and type text in manually it works as normal. Then I can switch back to the wysiwyg editor and the text will type as normal left to right. However, when creating a new block, the text initially will appear typing right to left so, as the person above stated, the word Mark appears as kraM.
Hope you all can fix this quickly.
thanks
hmark