The Bubble Text extends beyond the rightmost boundary of the Bubble when displayed in the Player. Has been seen in UPK 11.1 but not sure if it still happens in UPK 12.
You have set some text in the Topic (typically not in the Bubble showing the error) to be Non-breaking text. UPK somehow gets confused and starts applying nonbreaking formatting to other Bubbles.
Remove all non-breaking text from the Topic.