It's clear the 15 minute reset won't work in this case as the problem is that it runs fine at or near sea level (and possibly even OK once over 8K ft.?) but once back to sea level, it's screwed up. So it's the transitions from LOW to HIGH and back to LOW that the ECU is having trouble following. ... at least that's how I read the OP's description. A 15 minute reset would only get the bike running at the current altitude and once you ride UP and DOWN again, likely same problem will occur.

Usually turning off ignition, wait 30 seconds, then back on, will allow ECU to reset to current conditions or go back to default setting, so that's the quick and dirty temp fix.

But could well be a bad sensor somewhere or bad ECU ($$$$$) or moisture in there somewhere. I'm sure the dealer will happily sell you one ... but may be easier or cheaper fix? Their diagnostic computers should pick this up and they can read error codes too which should point to problem. Good luck!

Water certainly could have got into tank on unsealed fuel caps. Simply run something like Sea Foam or one of the moisture removing products to help. Can also help with Ethanol in the fuel. On KTM's it seems Ethanol can destroy fuel pump.

I run Sea Foam preventatively through all my bikes every few thousand miles.
Good luck, hope it's an easy and FREE fix!