I went back and read the the OP's information and your comments wanting me to reveal information I am not authorized to disclose.
Yes there is a problem with the transmissions popping out of gear.
The customer service sucks but there some responsibility at the end user - The other problem that has been glossed over - is that the bellhousing is not aligned with the engine. The centerline of the crank needs within 0.0010 inches of the centerline of the bellhousing. Having run 4 speeds for decades this most important measurement is always overlooked and makes a huge difference in how a 4 speed performs. Like night and day difference. I have always been able to get within less than 0.0010 inches. Proper tools and patience always win. We got one within 0.00005 but that took some patience.
There is no excuse for not checking the runout .on the bellhousing.
The side loading affects both the engine and transmission, causing crappy shifting, wearing out parts and eventually making the trans jump out of gear.
As to the 855's and other manual transmissions jumping out of gear or refusing to go into gear under load - i have to wonder how well the bellhousings are aligned. Then let's add the dimensional tolerancing issues with the gear sets. NEW gear sets that make noise/jump out of gear have parts that are not made correctly to the print - the parts do not measure to specification. Gear sets made properly work quietly so some one needs to ask the 3 parties making the transmissions about the lack of quality control. Is there a master gear set in house? Probably not based on the issues. I can't fix these issues but some one in the triumverate of 5 speeds - like Garrett Precision needs to get the gear checking metrology equipment out and make properly dimensioned parts. I can guarantee you they are skipping required manufacturing processes based on the problems. Call them up ask them what equipment they are using to check parts and show the paper trail required when making gears. They can't.
Metrology