What type of bug are you reporting? Mechanics
What is the bug?
Slow Start activates one turn too soon if the Slow Start Pokemon is switched in during the turn. On Showdown, this means switching in Slow Start lets it activate one turn earlier than it would have on cartridge. If the Slow Start Pokemon is switched in this turn, the counter shouldn't increment.
Are you able to replicate it? If so, how?
https://replay.pokemonshowdown.com/gen9customgame-2106755173
Tested and this has the same behavior in Gens 4, 6, and 7 as well, so probably safe to assume it's like this in all generations.
What is the bug?
Slow Start activates one turn too soon if the Slow Start Pokemon is switched in during the turn. On Showdown, this means switching in Slow Start lets it activate one turn earlier than it would have on cartridge. If the Slow Start Pokemon is switched in this turn, the counter shouldn't increment.
Are you able to replicate it? If so, how?
https://replay.pokemonshowdown.com/gen9customgame-2106755173
Tested and this has the same behavior in Gens 4, 6, and 7 as well, so probably safe to assume it's like this in all generations.