You can test that by renaming the backstab.2da in your override folder to something else (for backup). Then look for a version of backstab.2da inside the M&G/backup folders. Copy that one and put it into override.
If that solves the problem, you have an indication that M&G is the problem. If not, restore the renamed earlier file, (You may then use change-log to look for other possible culprits).
Thanks for the info to both of you! Good to know for debugging if I run into similar issues in the future.
If you installed the Feats component, then it will change backstab progression, by design. You can spend a feat to increase your multiplier.
From the Readme:
"A note about backstabbing: the base tables are simplified: x2 at level 1, x3 at level 9, and x4 at level 18. Shadowdancers' multiplier is 1 lower than other thieves. And Swashbuckers' multiplier is always x1. BUT, any thief can take the 'Backstab Bonus' feat once to improve the multiplier (to x5 for most thieves, x4 for Shadowdancers, and x2 for Swashbucklers). And Assassins may take that feat up to three times, thus reaching a x7 multiplier."
One of the reasons for this is to fix the issue of Assassins' bonus only kicking in at very high levels. This way, an Assassin can take the +backstab feat at levels 2 and 4, and be ahead of other thieves for the rest of the game.
That explains it then - thanks for the quick response!