Illustrator 2018 now skips artboards when using out of favor "Export As..."
There has been other issues reported about sizing which can be solved with a script (see other threads) but I have just encountered exporting of multiple artboards to PNG using "Export As..." where it skips some of the artboards -- it simply did not generate artboard no. 27 in my instance.
I get cynical at times but it does seem like Adobe deliberately destroy functions they want to phase out (in this case in favor of "Export for Screens"). If not then tell me why something that was working fine now is broken.