I only seem to have the issue while at work with bluetooth. I added the dongle, but still need to disable bluetooth to see if that helps. But quite frequently, while typing, suddenly it seems that the typing is interrupted and it doesn't continue with updating the things I have typed, then the last key pressed during the interruption is duplicated 50 times (I've counted, it's always 50 times) followed by the other keystrokes I have typed.
I have an MX keyboard at home, but still the M720 mouse, connected with bluetooth. I don't get the issue there, so it may be limited to the K850 keyboard.
I have not been able to test it well enough to confirm (it's not really my job, that's yours) but I think it happens less, or is eliminated if I put my watch and phone far away from my computer, so there may be bluetooth, or other RF, interaction between the keyboard and computer? That seems like poor design if that is the case, since phones and watches are fairly common things to have in close proximity, and other devices work fine with no interruption. Please let me know if this issue can be resolved through a software update.