El Capitan, the Finder, the file system and the built-in Rename feature.
I am in reading mode for many months now.
But I remember people having troubles when a slash appears in an item name (file / folder / both ?).
I am building a consistent list from magazine scans and so, sometimes I rename a bunch of files.
But I found a strange behavior: two files lost characters from the start of their file names.
I made a set of rename by 10 files and then I found where the offending error appears. I powered off my computer. Drink a glass of fresh water and come back and searched what lines (2) are the one with the problem.
I have found that “My title prefix 1/2 - suffix” and “My title prefix 2/2 - suffix” share the same properties / display the file name shrink. No slash in none of the other 50 or so items. (All are files).
So, I removed the slash and keep 1 and 2 to express part 1 and part 2 (I could use 1 of 2 and 2 of 2, too).
A rename on these two newly renamed files runs fine. All remainding lines were renamed as were the first other files.
My advice: even if the above is on El Capitan, do not use a slash anymore in your items names to avoid potential troubles.
I hope this will help.
BTW: if your experience is different on Catalina / Big Sur (all flavors), don’t be shy and express yourself. 
PS: Xojo was not used in the above, but is below