We purchased two 3821x - Streaming stick 4k+ remotes in January. In October one of the remotes began changing the volume with no human interaction. I contacted Roku support and received a replacement.
The replacement worked fine for several weeks. Both the replacement and the original purchased in January have intermittent function with both volume and power button being able to control televisions.
Remote 1
Model 3821x - Streaming Stick 4k+
Serial # X01700SWCJD5
Software version 11.5.0 * build 4312-CU
Television LG 43UN7000PUB
Remote 2
Model 3821x - Streaming Stick 4k+
Serial # X01700UEE5RT
Software version 11.5.0 * build 4312-CU
Television Samsung LN40D550K1FXZA