When you configure connectivity check with a positive timeout, the check should happen periodically. It doesn't due to a bug.
Broken in 1.8.0.
Need backport: https://cgit.freedesktop.org/NetworkManager/NetworkManager/commit/?h=nm-1-8&id=e984d9eb36f7838df58c0606bd00efc10730d329
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.