I encountered this a few time in some books.
It has something to do with what is being used as a space in those ePUBs.
It will not be visible when reading the book but there is something else being used instead of a space. If you use calibre and edit the book it shows up as these gray boxes in calibre (refer to screenshot below).
The first screenshot shows the problem and last screenshot is from Calibre when editing the book.
You can fix the the problem with the book using calibre before importing by using using “find and replace” to replace the gray box with a proper space.