Water can't get in.

As Jim says if water could get in air would of gotten out long before you washed it.

Stan's instructions say - Important: While shaking and pouring sealant, the bottle must be turned upside down. (See photo 5). The sealant contains micro sealing particles; these particles need to be suspended in the sealing fluid for maximum performance.

And you should be shaking pretty hard. Is it possible you've got more fluid than particles by not shaking enough? Left alone Stan's separates. I'm thinking if you don't get more liquid separation in the tire it might not coat properly and then you might get some of the rubber washed off and mixed in - dirty water maybe?