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I believe this is not a bug, but I had no time to further investigate this so far. Definitely not a regression - code from the original fix is included in RHEL 7 coreutils - but further extended by upstream.
RHEL 7 df uses extended logic to determine which mountpoint should be shown (from multiple possible options). As Ondrej worked on the deduplication, setting needinfo on him, but I tend to not a bug this request (we have multiple bug reports about deduplication improvements).

As per conversation with Ondrej, we found out the patch we applied to rhel-6 is missing in 7. We further discussed, that it might be better to patch rhel7
with the upstream version of get_disk(), with the last change in the
dbd7c9452 upstream commit.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2015-2160.html