Minecraft FTB Revelations server does not automatically use the S:mobGriefing=false rule, i have found that manually using the mobGriefing rule makes the other rule work as expected (i use this mod to bypass villagers being disabled from farming when using the command). This however is not the same as in single player, in single player it accepts the config file settings.

Minecraft version: 1.12.2
Forge Version: 14.23.4.2759
FTB Revelations: 2.5.0
BetterMobGriefingGameRule version: 7.0.1.j

P.S I dont think it has anything to do with FTB i just provided the info as a precaution just in case the FTB server files add something that conflicts)

Thanks for the report, I'll investigate when I get chance.

My initial thoughts are that it is one of the following possiblities.

1. The world is not a new world. The default mobGriefing values for existing worlds is never changed, it
   is only ever done on creation. The exception here is any that are missing (such as the new entity based rules) which will be added the first time the world is loaded with the mod installed.
2. One of the mods in FTB Revelation is changing the value after I set it upon world creation.
3. The technique I use to determine world creation is not reliable enough.

By far the most likely is number 1, if you can confirm that is the case then I could instead look at adding an additional configuration flag to force the values upon any world start instead of only during world creation.

ah, i didn't realise the test command for finding out if mobGriefing was on or off, i used the good old fashioned spawn some creepers and see if they damage terrain.

the location is in \FTB Revelation\config, and i originally used 1 from prob a few versions back and i initially thought that it may have changed so removed it, let it gen a new 1 (it did in the same place) and could not see any changes but edited that instead.

I've created a local server with FTB Revelations and BetterMobGriefingGameRule with the same versions as you reported and all of the behaviour is what I would expect. I followed the following steps to verify.

1. Start fresh FTB server and let it fully load.
2. Run "gamerule mobGriefing" on the console - default rule = true, entity rules = inherit.
3. Update the config file to set default = false and villager = true.
4. Restart the server and let it fully load.
5. Run "gamerule mobGriefing" on the console - default rule = true, entity rules = inherit.
6. Stop the server.
7. Delete the world.
8. Start the server and let it fully load.
9. Run "gamerule mobGriefing" on the console - default rule = false, villager rule = true, other entities = inherit.

What is the location of the config file you're using and have you edited the default generated one or created your own from scratch?
If possible could you attach your config file so I can rule out any issues with the config itself.

The next steps after that would be to add some additional logging to the mod to try and track what is going on.