When you create a Mention of an Entity, not only does it automatically add an actual “space” into the text after the Entity Mention (OK, that makes some sense), but there is also essentially some padding around the Mention, particularly at the end. Here you see an entity mention with no text space, this is all just the “padding” or whatever that is rendered with the entity:
IMO I should be able to squash entity mentions together with the text that follows them if I want to! 
The underline differentiates them enough for me. So basically I think the end of the entity mention should act just like any other character. Have the default text space inserted after, that’s fine and makes some sense, but this non-removable space is 
Yes, this is a very minor but for me very annoying thing. There may be debate about whether this is a “bug”, but I’m more sure it’s not a “feature”. 