Yes! This is normal and a good sign the soaking is doing its job.. They will pee and they will poop when you soak, its a given. Pretty nasty, but a good sign. Just make sure you change the water when your tort does this and continue the soak
Yes, being in the water stimulates the tortoise to urinate and refill on fresh water but I wouldn't say that soaking is a stress free experience for them, especially if they're new to it and unsure of the whole experience/process. It has made me question the frequency of soaking, primarily for my Russian tortoises. I had found that their urates were a good consistency with just a water dish in the enclosure. I began soaking them, thinking it couldn't do any harm and found that they would always urinate and defecate during their soaks but that they wouldn't drink while in the container. The urates started getting harder and grittier. When I stopped and let them self regulate their hydration (no soaks) the urates returned to normal.
Especially for tortoises new to soaking I think there are several elements that could potentially cause them to defecate and urinate from being stressed during the process. Once they become used to it, maybe they are less surprised and more comfortable.
1. Being carried to where they will be soaked
2. Being confined in a relatively small container
3. Being in water that they can't get out of (whether it's only up to the bridge of the shell or up to their neck)
4. Experiencing a sudden temperature change (depending on the temperature of the water)
5. Constantly trying to climb up the sides of the container they are being soaked in
For my Sulcatas, it's a different story. Soaking really has always had positive effects. They seem to be more courageous about drinking when I'm around and while soaking.
My Tidgy used to pee in her soak water for the first few weeks I had her, but has never done so (to my knowledge) since.
She does pee, of course, but never in her bath water.
I don't bath my torts, they have a water dish to soak and drink whenever they want to, which is at least once a day. But they never pee or poop in there, I guess it makes more sense for them to save it up for their hide (for more humidity).
I noticed when I moved my sullies to a pure grass/mazuri diet, the urates stopped. When I re-introduced some green leaves (even if it's just 10-20% of their diet), the urates came back.
So yes, it's good that your sully is expelling all of those. But if you want it to stop, I recommend you try a pure grass diet.