Rising Star is a relatively new Pokemon Go medal that, as per the in-game text, is rewarded for defeating 2/10/50/150 different species of Pokemon in raids.
However, I recently defeated a level 3 Mawile Raid. The recorded number of species I had then increased from 17 to 18. This was despite previously defeating level 2 Mawile raids prior to when the medal was introduced. Following the in-game text, I would not expect an increase from clearing the level 3 Mawile raid.
So what does count as 'different species' of Pokemon for this medal? I suspect that the medal either counts different raid levels of Pokemon as different 'species' or that only raids from a certain date onwards are counted, though it could be something else instead.