That’s certainly the issues that I had back in my Frankenstein Gen8 days.
Beveled gears naturally want to walk over the top of each other under load.
My poor suffering 480-490 FPS Nylon boxes needed plenty of additional extra threaded bolts throughout the sides of the gearbox halves!
Even when shimmed “tight” under no load, I still experienced this same issue of flex allowing gears/bearings to depart the locating pockets.
I even resorted to adding “packing” between the gaps between the gearboxes and receiver housings to try and hold everything together!
It always came down to spring rates at the end of the day. I was trialling F100 Truck springs to achieve 480-500 FPS out of a nylon Gen8 box!
All good fun at the time…but I’m sure that @Echo3319 ‘s problems are from other issues than M190+ spring rates