Yes, in that project I have over 80 routers (mix RUT230, 240 and 955).
I've never observe value higher that 51 -dBm in any router (data from last 7 days confirmed by zabbix)
Average values are <-82, -56> dBm
Yes, for these case after router get -51 dBm it tries to ping google DNS and reboot after that (auto-reboot - not succeed). Router uptime is reduced after that (as a confirmation of reboot).
I have confirmation that state for two devices. It take time to connect again (even few hours) - for RUT230 only with FV version at main post.
Right now I have situation with one of RUT230 that is offline and the last signal strength value is -51 dBm (previous reboot was related with same signal strength value as you can see on diagram set at imgur).
Currently I've check history for last 7 days and three routers behave the same that reach signal strength -51 and router uptime is reduced.
For one router RUT230 I have situation that average value is -51 (but there are some points with -55 dBm) but router works properly (RUT2XX_R_00.01.11.2).
With two routers I have constant -51 dBm with no other values (which is strange) but also devices works properly (RUT2XX_R_00.01.10, RUT2XX_R_00.01.11.2).