Windows Explorer Bug? Why is Windows Explorer Search Box not recognizing the character “R” for a set of file names?
As a workaround, search using lowercase.
You may have stumbled upon a Windows Search bug. Explorer appears to interpret the OR within the search term as the OR operator (even though it doesn't have spaces around it).
The same behavior also happens with NOT and AND in the search term.
Advanced tips for searching in Windows
Adding operators
One way to refine a search is to use the operators AND, OR, and NOT. When you use these operators, you need to type them in all capital letters.