check the forums, this has been a thing for a while. You can check the damage you've taken and gauge the health of the enemy based on the damage you do.
for example, a difficulty of about 40% will feel like it always does. But, when you make a squad, especially a 4 man (at least a group of mostly 100+), you will notice the enemy AI become bullet sponges and do WAY more damage. BUT, when you get to the finish screen where it tallies your points you will see the difficulty is at 40%, but clearly had the enemy values at what would normally be at 50% or higher. But it just says 40% difficulty. It wouldn't be SO bas if it said "50% squad bonus to difficulty" at the end because then the system would REWARD us for 50% difficulty. The problem is it's rating us at a 40% difficulty when we worked at a 50% difficulty.
I stopped joining groups after this happened REALLY bad one day with a full squad. We kind of notice until after a few games and then we found it odd that a bunch of veteran players from level 150 to 350 were getting trashed by missions with difficulties, it said, in the 40ish %. After the third game the group got really demoralized, and we broke group under very bad air, and we never grouped again. And I still won't until MSE says they've resolved this issue or correct it to properly display the difficulty and admit the game raises the difficulty for squads.
I strongly believe that if the game tells you it is at a certain % difficulty enemy values should be the same at that given difficulty regardless if a player is playing solo or in a premade group. In other words, if a wardog mook has 200 health at 40% when in solo queue, he should also have 200 health at 40% in a group. NOTE: I'm not talking about MY MMR, but the average difficulty % the game creates for matches that is displayed at the end of the game.