You should uninstall haproxy 1.5 cleanly, so that when you do make a mistake while installing the new release, you get a proper error message instead of strange behavior.
When you did that, double check that there are no files like /usr/lib/systemd/system/haproxy.service, haproxy and especially /usr/sbin/haproxy-systemd-wrapper.
Please don’t do copy the executable around, what’s wrong with /usr/local/sbin/?
Once you have a clear situation, install the system unit file that ships with haproxy as per:
Once you have done all this, follow the procedure here to setup systemd: