good tech should sort it out, be patient. i had a similar issue w mine w/ lots of middle keys sticking and failing to reset for a restrike. required taking action apart and replacing jack and whippen (spring should be find but could be a culprit, or anyo other number of things). it took several months and many appointments to sort most of them out. your millenium action should not be as suseptible to humidity, actually kawai in general do better than other brands in high humidity climates/envrionments. but it's a super complicated deal, each key can impact a hundred different tiny parts so it's not unusual that as the hours build up you'll discover a few places that require a bit of fine tuning and attention
I wouldn't be able to breath without coughing at that low humidity. It is more like 80-95% around here all the time, with ittermittant sixties in the winter when the cold fronts blow in. Like today, when I am coughing. My pianos made it through the summer at 95-100%, but I did manage to buy a room air unit in the September closeouts so maybe I can keep the humidity down some next year. I hope conditions are right when the tech comes to where the problem repeats itself. Nothing like trying to get an ittermittant problem repaired when it doesn't show up in front of the tech.