Given these devices:
network:pingdevice:Phone1 "Phone 1" [ hostname="172.16.0.239", retry=1, timeout=5000, refreshInterval=300000 ]
network:pingdevice:Phone2 "Phone 2" [ hostname="172.16.0.233", retry=1, timeout=5000, refreshInterval=300000 ]
network:pingdevice:Phone3 "Phone 3" [ hostname="172.16.0.133", retry=1, timeout=5000, refreshInterval=300000 ]
And these items:
DateTime Phone1_Lastseen "Last seen [%1$tA, %1$tm/%1$td/%1$tY %1$tT]" {channel="network:pingdevice:Phone1:lastseen"}
DateTime Phone2_Lastseen "Last seen [%1$tA, %1$tm/%1$td/%1$tY %1$tT]" {channel="network:pingdevice:Phone2:lastseen"}
DateTime Phone3_Lastseen "Last seen [%1$tA, %1$tm/%1$td/%1$tY %1$tT]" {channel="network:pingdevice:Phone3:lastseen"}
I get these logs:
2019-03-12 12:48:58.123 [vent.ItemStateChangedEvent] - Phone1_Lastseen changed from 2019-03-12T12:44:40.895-0400 to 2019-03-12T12:28:58.120-0400
2019-03-12 12:48:58.124 [vent.ItemStateChangedEvent] - Phone2_Lastseen changed from 2019-03-12T12:14:40.297-0400 to 2019-03-12T12:28:58.121-0400
2019-03-12 12:48:58.125 [vent.ItemStateChangedEvent] - Phone3_Lastseen changed from 2019-03-12T12:14:40.298-0400 to 2019-03-12T12:28:58.122-0400
Note that Phone1 is present while Phone2 and Phone3 are not. From the command line, ping, arping, all work as expected. Note that Phone1 lastseen is being changed from a last correct value to a (wrong) value in the past, which matches the same (wrong) time stamp set to all 3 devices. Phone2 and Phone3 have been disconnected for several hours now and at different times.
This discussion talks about issues with pingdevices but there is no indication if it is a configuration issue or just something not quite right with the implementation.
Where do I even begin looking? 
Ubuntu 18.04
$ openhab-cli info
Version: 2.5.0.M1 (Build)
User: openhab (Active Process 9414)
User Groups: openhab tty dialout audio bluetooth