According to one source, the 921 bulb draws 1.2 amps at 12 VDC. With 10 of them on one circuit, you could be at around 12 amps. Most "small DC off/on switches" are rated at 10 amps, so the switch "could be" overloaded..... Here's one chart for checking bulb specifications:
https://www.rvledbulbs.com/v/vspfile...scentbulbs.pdf
I'd suspect that Keystone, when designing the trailer does the same with electrical load that they do with tire load.... Weigh the trailer, deduct the tongue weight and what's left, divide by 4 to get the tire load. That doesn't account for "side loading" nor for any of a number of other conditions, many of which could lead to an "overloaded tire" at a specific wheel location.
Same with electrical load, if they "calculate" that the "average" camper would only turn on half the lights at any "given time", so they calculate the load at 10 total bulbs, only 5 turned on, so only 6 amps of use for the "average"... Even though we all know that in some situations, all of them would be turned on, thereby overloading the switch and possibly the wiring, depending on how long the wire run actually might be.
As sourdough said, convert to LED's and solve the problem without the hassles of trying to figure out what the heck Keystone did or how they did it.....