I’m running a mosquitto broker v1.4.14 on a DietPi Stretch setup. I suspect the problem was as above. DietPi picked up 1.4.15 from mosquitto.org but Debian is still (or was) at 1.4.14 (which works fine). 1.4.15 did not install due to a dependency conflict. I’ve not tried it since, but I think DietPi may have reverted to the Debian Repo.
As discussed elsewhere, the actual startup of the service may cause an issue if the mqtt_input service starts before mosquitto. I’m not seeing that, as my broker runs on a different machine.