Oh really?
Master Yi just got his W AP ratio nerfed from 30-60% per second down to 15-30% per second. According to Meddler, right here
This is a tooltip correction only. The actual ratio's been 0.15 for a long time.
(Emphasis added)
Or look at Sion's Q: For several months (about 5 if I'm not mistaken) his tooltip erroneously stated that his Decimating smash dealt 80% damage to minions and monsters. This was in fact not true, and apparently hadn't been for a very long time, possibly since the last time his Q was ever mentioned in the Patch notes back in 5.6, maybe even earlier. According to Riot Scruffy, right here
Unfortunately this is a tooltip bug. I'll double check next week and make sure that the intended numbers are working but my hunch is that 60% is the real number that he is balanced around.
That thread about Sion was from 4 months ago. The fix never showed up in patch notes and only just recently showed up in the Wiki, and as far as I can tell that is just because somebody went to the trouble of calculating the actual values and typing it in, not because Riot fixed the issue.
These are only two recent examples, there are undoubtedly several more, some possibly larger than this. THAT is why I would not be surprised by this being a tooltip bug and an undocumented nerf.