A toilet plume is the invisible to the naked eye, cloud-like dispersal of raw, infectious sewage particles as a result of flushing a toilet. Flush particles rapidly rise out of the bowl and several feet into the air after flushing. Aerosolized droplets can remain suspended in the air for tens of minutes between users, posing an inhalation risk. These invisible particles go on to coat and spread onto surfaces like floors and counters or objects like hand towels, bathmats, or even toothbrushes.
Here's a question: Do they go straight for it, or do they only seem to go for it when the toilet hasn't been flushed (but unused) for a while?
There may be some truth to the idea that cats can taste the additives that are put into municipal water (in developed countries anyway), and it takes a while for those to dissipate once the water is out of the pipe.
Often, the only significant quantity of water that has sat for long enough is what's found at the bottom of a toilet.
Alternatively, if you have a water filter for drinking water, and that makes it into your cats' bowl(s), it might be imparting a flavour onto the water that you can't taste but your cats can. Chances are your toilet doesn't have the same filter.
The "cure" might be to provide water that's not straight from the tap (faucet) or filter, and maybe in a different bowl/location in case they've developed a negative association with either of those.
But yeah, keep the lid closed on the toilet regardless.