32768 gets clamped to 32767, so you’ll get the same result using either (and if you enter 32768 in live mode and then press <arrow up> to see history it’ll be changed in history as well).
adding 1 to 32767 results in overflow and due to binary signed arithmetic you get -32768. so doing either one of these:
+ 1 32767
+ 1 32768
Y 32767; Y + Y 1; Y
Y 32768; Y + Y 1; Y
will give you -32768 (so in the L bug both 32767 and 32768 will cause the issue).
999999 will get clamped to 32767, adding 2 of those will result in overflow with the result being -2, as expected.
it increments I in each iteration, so what you’ll get will be 1, 4, 7, 10 (since the loop itself will increment I by 1 and then we are adding 2 on top of that). you can test it easily by doing L 1 10: P O I; I + I 2 and then checking pattern bank 0.