Some more info for you on calcium…humans, cats and dogs all poop out excess calcium which is why you’ve never seen this before. Rabbits also have a different shaped bladder where for some reason the out hole (I’m not great with anatomy, the urethra I guess) is at the top of the bladder. So the calcium can build up in the bladder and cause issues. One way to ensure that doesn’t happen is to make sure bunny gets lots of play time to move and binky so they’ll shake up the sediment in the bladder so they can pee it out.
It’s normal for their pee to always be slightly cloudy and for them to excrete larger quantities of calcium on occasion. If you rabbit consistently has thick urine you should consult a rabbit savvy veterinarian. There is don’t disagreement on whether calcium in the diet actually causes complications but if you’re concerned about it you can try reducing the amount of calcium in your bunny’s diet by avoiding greens with the highest calcium content like spinach and kale. I filter my bunnies’ water because we have very hard water with a lot of calcium in it and I would rather be safe than sorry.