No Futaba RX streams RSSI that can be read real time. Some aftermarket devices, like the one you found, read something, they call it RSSI, but I cannot say for certain. Again, the issue is with the data some of these devices provide there is no clear path to interpreting it.
Having said that, if you log your telemetry you can view it using the converter and there is a column that gives an indication of the telemetry signal strength back.
I always suggest using a preset failsafe so you can know for sure if you had a hold. I prefer this instead of holding last good command. Failsafe places my controls in a known position, usually neutral.
How many times did you have an issue? Any commonalities between the times it happened?