I had the same issue (mbp 15 tb) and was provided with a replacement which took awhile. I experienced the issue when I started coding a week after I received it, which ended up slowing me down considerably.
The new replacement is working much better (at the moment).
However, I still believe (and I'm probably wrong hahaha) this is a design flaw when the keys are this small with the new butterfly mechanism, because no other key on this keyboard fails to register when hearing/feeling a click. I can still replicate the issue with the arrow keys on the new one if I'm SUPER gentle (feeling/hearing a click with no change to the cursor). These new keyboards feel amazing, aside from the arrow keys remaining a possible issue for many users.
What I'm hoping is that the severity varies across units is an issue at the start, regardless of how many times we've pressed the arrow keys.
I'm also wondering if somehow it gets worse the more often we close our lids and depending on the amount of pressure applied when it's closed during travel and whatnot. I realize that sounds absurd, but I can't think of anything else within the normal operation of the mbp that would apply any pressure on keys aside from how hard/softly/often we press the keys.
If it starts acting up again then I'll know that the responsiveness of the new butterfly mechanism diminishes with time due to normal use of the mbp, which would be highly unfortunate.
Since I'm outside the 14-day window, and repairs (according to someone at Apple that issued me a CRU) on this newest generation apparently don't begin until the end of December (around this time) anyway, so it appears I'll need a contingency plan (time machine often and an older backup mbp) if it becomes an issue so it doesn't slow me down in my work.
If it does happen again, and if they send me a replacement (CRU) instead of repairing it, I will probably sell the replacement and save myself the grief and time. My configuration isn't standard (CTO) so entire replacements will likely have to be delivered from Shanghai instead of a stockpile somewhere in the states, which will take a lot longer. sigh.
I'll make an update in about a month or sooner depending on whether or not I experience diminished responsiveness with the arrow keys.