My GFCI/AFCI 20a kitchen socket warning lights are both flashing
Stop throwing parts at it and actually troubleshoot
It would help to read through your GFCI/AFCI instructions and get clear on whether the device is indicating GFCI or AFCI fault.  In other words, treat the indication as if it is genuine, and not just gibberish from broken junk.
Let’s ditch the red herring: it ain’t the MWBC
We know it’s not the MWBC (Multi-Wire Branch Circuit).  First, there is nothing wrong with MWBCs in general, but they do require special handling for GFCI and AFCI.  The idea of an MWBC is to share a neutral, and that is incompatible with two 120V sub-circuits each having independent GFCI devices.  The neutrals in each GFCI’s protected zone must be kept separate from any other; and travel only with the protected partner hot.
It’s no problem to do this with MWBC wiring; the two sub-circuits simply must diverge and never share a neutral after the GFCI(s).
We know your kitchen is wired correctly because if it was wired wrong, it would instantly trip with ANY use on the shared GFCI downline, and that is not the symptom.  Also it was wired by a pro, who would know better.
So it’s not an MWBC problem.  MWBC is simply something you must be aware of while working on the circuit.
Whenever a GFCI trips, immediately check appliances
The first thing you must do is unplug everything on the downline of that GFCI.  See if the problem clears.
Meanwhile, take those appliances to a distant unrelated GFCI and see if they trip it there.  In this case you’ll also have to test for AFCI trip, so try it on an AFCI also.
Next, check the downline
Remember, we’ve already pulled all the appliances on the downline, so that’s not it.  It must be in the wiring.
If that does not clear the problem, we pull the GFCI out and remove the wires on the LOAD terminals.
If you can’t abide losing power to that downline, temporarily move it to LINE.  However make sure to put it back!
With LOAD terminals empty, and nothing plugged into the GFCI, it must hold and not trip.  If it trips, the GFCI is defective (not likely).
Since the GFCI did not trip, we are back to troubleshooting the downline wiring.  Follow it, then divide, and conquer.  Reconnect it to LOAD, split what remains of the circuit and see “which side” the problem is on.  Rinse wash repeat until you found it.
AFCI trouble
Since these GFCIs are also AFCI (what’s the point of that?) the AFCI will detect wiring faults in the downline, or even in the supply/upline to some point.   The usual suspect is “back stab” connections at receps: convert those to side screws.  Make sure the side screws are all “run down” (not sticking out, they come stuck out by default, and backstabbers ignore them because backstabbing is all about being in a hurry)... and aren’t hitting a metal box or a ground wire, e.g. as a plug is wrestled in/out.
Since AFCIs literally “listen” to the wire for the crinkle-crunch sound of electric arcing, it’s possible for them to hear that on the upline.  So an AFCI trip might result in wiring problems on the LINE side of the AFCI, back to the breaker and neutral bar connections, or even a loud arc fault on another circuit!
AFCIs are not required in kitchens, and if they were, AFCI receptacles are an unacceptable shortcut unless the wiring to the panel is in metal conduit.   So in all seriousness, getting rid of the AFCI function is an option.