Not every bug results in the program doing the wrong thing, they often just make the program do the right thing very slowly.
And nobody notices, since it still produces the right result.
Now the bugs that get ignored for new features cause bad results AND bad performance.
If the stream is buffered, then all operations, including fread, are supposed to go through the buffer.
All three of these should issue buffer-sized reads to the operating system:
1. A loop which calls getc(stream) 65536 times.
2. fread(buf, 1, 65536, stream)
3. fread(buf, 65536, 1, stream)
The more direct behavior of fread should only kick in if the stream is configured as unbuffered.
I would say that the way low-level reads are issued to the host operating system is a "visible effect" of the program, so I suspect this may actually be a matter of conformance. I.e. it's not okay to issue those reads however the stream library wants as long as the data is read.