Yup, you're putting 40A on that poor neutral wire.
There's another on bottom right with the same problem
Note the red and black on #6 right. That looks to be another MWBC on a duplex (the very bad thing). Well, I'm not sure of that. Perhaps the last guy was clever and the red's partner is actually the lower black on #5. Still no handle-tie, and my fix-it advice assumes it's the danger-wrong combination, so check it out, and if necessary make it so when folowing my advice.
And you also know that the one the 4th column on the right used to be one, although both are on the same breaker now (codevio?) but at least it won't overload the neutral.
Oh, and one more thing. You also have two more pair of MWBCs in spaces 1-2 right. See that? No handle ties. That needs to be corrected as well, but that's easy - just swap them for a $20 BQC215215 tied quadplex. Follow the wires back to their cables to make sure the inner wires come from the same cable. Because you need to tie the wires in an MWBC, don't cross them!
I gather the way this happened is the "contractor" was not a competent electrician and was there to sell you something else, and just rearranged the furniture as needed to make his AFCI stop tripping, without giving a damn about what he left behind. This is a genuine hazard, and you might want to warn your town's inspector to keep an eye on this guy. If he wants to see it, this question should suffice. Fix this ASAP! No delay.
By the way, notice how they double-tapped the AFCI breaker? Are Bryant breakers listed for 2 wires on a terminal? I don't think so!
Each of these MWBCs also needs handle-ties. You see the quadplex breakers on the left? You can get quadplex breakers in two flavors. One has a 2-pole factory-handle-tied set on the inner pair (just like yours) but the outer pair are independent. The other has that and also has factory handle-ties on the outer pair, making it a viable substitute for two 2-pole breakers. You'll need this last kind quite a lot.
Option 1: Minimum possible: keep CAFCI as it is
In this case, you move #3 right's wires to the inside of a quadplex 20/20 common trip such as a BQC220220, as mentioned above. You pull off breaker #6 and move breakers #4 and 5 down 1, so space 4 is open. You put the wires off breaker #6 onto the outer screws this new 20/20, and snap this common-quad into spaces 3 and 4. Whew!
Then replace the unit in space 1-2 with a BQC215215 tied quadplex as mentioned.
Option 2: Damn the Code, eliminate the CAFCI
In this scenario, you undo what they did with the CAFCI and put it back to the way they found it, except with the proper handle-ties this time. You yank the AFCI and again a BQC220220 goes in spaces 3-4 (right). Space 3's wires go to the middle terminals, space 4's wires go to the outer terminals. Neutral back to the neutral bar.
And... replace the unit in space 1-2 with a BQC215215 tied quadplex as above.
That was easy.
Oh. Wait. There's still the rogue MWBC on space 6 (right) with its red and partner black (wherever that is). Those two need to be in the middle of a quadplex. But what goes on the outside? I see the two independent circuits on the 15-20 duplex right above, but they don't make 15-t20-20 open quadplexes. I suppose you could downgrade the 20A circuit to 15A protection, but that could be its own codevio. Damn.
OK, the only option I see is to replace the 50A breaker on the top left with a quadplex 2-pole 50A + 2-pole 20A, such as a BQC220250. The fat load goes on the 50A terminals obviously, and space 6 Red and its partner go on the 20A terminals. And now there's an empty space in the panel. Fill it with whatever breaker you have lying around, just mark it "no connect".
Option 3, replace the bleepin' panel
I was looking for a way to use the spare space to allow a 2-pole (2-space) AFCI so the disposal and dishwasher could be separate again, but it's bleepin' impossible, this panel is just too tight.
All this is stupid. You have already downgraded your circuit capacity (possibly illegally, can dishwasher and disposal be on the same circuit??) and this is only the first time you have encountered the "AFCIs/GFCIs don't come in double-stuff" problem. This is going to keep coming back at you.
So slap yourself on the back, you endured the electrical equivalent of a firewalk, you had a 12-space panel and lived to tell the tale. You can keep your grandkids up at night with your stories.
Now it's time to get a proper panel. I would stick with Eaton and a panel with the top and bottom holes in the same positions. See if you can get Eaton CH since it crams 25% more breakers in the same height. This looks like a turkey-shoot DIY job, since the main breaker is upstream of here so you can de-energize the whole kaboodle. You will need the pull the wires back out of the 2 lower conduits far enough so you can cut the conduits shorter. But it should be straightforward, hand hacksaw then deburr the inside. Reuse the same fittings.
One other defect.
Being a subpanel, the panel needs to have absolutely separated neutral and ground bars. Now they are mostly separated. Fix that now.
I don't see a ground wire, so I expect ground is brought via the metal conduit. That is fine.