A reddit user took a look at Apple’s Butterfly keyboard and searched for the cause of possible defects.
Here you can find the complete reddit thread incl. all pictures of cil3x.
cil3x worked for an Apple Authorized Service Provider and started to disassemble the keyboard with the butterfly mechanism, which are used in MacBooks since 2015. So much can be revealed: he couldn’t find a clear reason for the partial failure.
The keyboard is complex and seems to have a stable design. The writer’s goal was to refute the dust theory, i.e. to make it clear that too much dust cannot be the sole reason why the keyboards fail. His experience shows that especially frequent writers have to struggle with defective keyboards. So it could simply be mechanical wear and tear.
Other theories have already cited temperature and temperature changes as possible reasons. In addition, you can read in the comments that the geographical location with sea level and humidity can have something to do with it.
Stable designed, but not robust.
Our experience with this keyboard design is 50:50: a first MacBook Pro from 2015 still works perfectly with the first keyboard and is in daily use. A second has already received an exchange. It seems Apple has really thought through the design of the current keyboard quite a bit. However, the keyboard is probably very strongly optimized and doesn’t have much fault tolerance with regard to heavy use or various external influencing factors. The further away you are from the sweetspot (normal temperature, no CPU load, average typing behaviour), the more likely defects can occur. The keyboard does not tolerate continuous use, high temperatures, eating at the computer etc. for long.
The butterfly keyboard seems no longer as robust as it was in the older MacBooks until 2015. It is by no means cheaply constructed, but perhaps even so high-tech that Apple probably didn’t expect so many problems to occur in everyday life. These often accumulate among frequent users and users whose computers often reach high temperatures.
