I think there are two separate issues here.
The first one is that sometimes when using multiple emojis without spaces, some of them won’t convert correctly. Could this be due to:
(no spaces between)
(spaces between)
The second issue is that extra square after some emojis. Not sure yet what this might be.
Update: those extra squares only show on Safari, screenshot below has the same post in Safari, Chrome, Firefox (left to right):
