Why is xargs not replacing the second {}

Question

I'm using xargs to try to echo the name of a file, followed by its contents. Here is the command

find output/ -type f | xargs -I {} sh -c "echo {} ; cat {}"

However for some reason, the second replace after cat is not being replaced. Only for some files, so some files do work correctly.

To be clear, I'm not looking for a command that lets me echo the name of a file followed by its contents, I'm trying to understand why this specific command does not work.

Answer 1

Turns out that the command was too long, so it was working with shorter file names and failing for longer ones. From man xargs

-I replstr
             Execute utility for each input line, replacing one or more occurrences of replstr in up to replacements (or 5 if no -R flag is specified) arguments to utility with the
             entire line of input.  The resulting arguments, after replacement is done, will not be allowed to grow beyond 255 bytes; this is implemented by concatenating as much
             of the argument containing replstr as possible, to the constructed arguments to utility, up to 255 bytes.  The 255 byte limit does not apply to arguments to utility
             which do not contain replstr, and furthermore, no replacement will be done on utility itself.  Implies -x.