It started happening to me (951G-2HnD, 941-2nD) on the latest stable ROS 6.45.1. Ethernet and ppp links.
I had may be the same issue last week or two. 951G-2HnD and hAP ac lite (6.44) had lost internet connection and was rebooted manually by users.
I found this in hAP logs:
Jul/12/2019 07:37:24 wireless,info 90:A4:DE:EC:9D:B1@wlan1: disconnected, received disassoc: sending station leaving (8)
Jul/12/2019 10:04:30 wireless,info 3C:F7:A4:ED:E4:29@wlan1: connected, signal strength -87
Jul/12/2019 10:04:35 wireless,info 3C:F7:A4:ED:E4:29@wlan1: disconnected, unicast key exchange timeout
Jul/12/2019 10:09:26 wireless,info 3C:F7:A4:ED:E4:29@wlan1: connected, signal strength -86
Jul/12/2019 10:09:31 wireless,info 3C:F7:A4:ED:E4:29@wlan1: disconnected, unicast key exchange timeout
Jul/12/2019 10:09:59 wireless,info 3C:F7:A4:ED:E4:29@wlan1: connected, signal strength -86
Jul/12/2019 10:10:04 wireless,info 3C:F7:A4:ED:E4:29@wlan1: disconnected, extensive data loss
Jul/12/2019 10:14:47 wireless,info 3C:F7:A4:ED:E4:29@wlan1: connected, signal strength -88
Jul/12/2019 10:14:52 wireless,info 3C:F7:A4:ED:E4:29@wlan1: disconnected, unicast key exchange timeout
Jul/11/2019 17:35:21 system,error,critical router was rebooted without proper shutdown
Jul/11/2019 17:35:24 pppoe,ppp,info pppoe-dom.ru: initializing...
Jul/11/2019 17:35:24 pppoe,ppp,info pppoe-dom.ru: connecting...
Jul/11/2019 17:35:29 bridge,info hardware offloading activated on bridge "bridge" ports: ether2-master
Jul/11/2019 17:35:29 bridge,info hardware offloading activated on bridge "bridge" ports: ether3
Jul/11/2019 17:35:29 pppoe,ppp,info pppoe-dom.ru: terminating...
Jul/11/2019 17:35:29 pppoe,ppp,info pppoe-dom.ru: disconnected
Jul/11/2019 17:35:29 pppoe,ppp,info pppoe-dom.ru: initializing...
Jul/11/2019 17:35:29 pppoe,ppp,info pppoe-dom.ru: connecting...
Jul/11/2019 17:35:29 bridge,info hardware offloading activated on bridge "bridge" ports: ether4
"Jul/11/2019 17:35:21" was real local time at reboot moment. Any ideas what has happen?
It seems that wrong time was the reason of their zombie mode. Clock settings was default.