Yes. Spa bather load is immensely different than pool bather loads in terms the amount of waste generated (sweat, saliva... other unmentionables) versus actual amount of hypochlorite available because of a very low water volume. It's like having 400 people in your 30,000 gallon pool at the same time with water temperatures in the 100s. This is why it's necessary to bring FC levels up to over normal pool maintenance levels after soaks. Usually after a soak, nearly all of the FC is consumed so there's actually an FC debt. It can be thought of as having a negative FC, where there is more bather waste than the current FC can handle and you must target a higher FC just to get things back to normal maintenance levels.
If 4 people soak in a tub for 1 hour, you're going to have FC debt, unless you started with shock levels of FC. Targeting a shock level FC will overcome that FC debt and leave your FC likely above the minimum FC once again.
If there were a lot of people in the spa for a long time. I'll dose the spa, then recheck after 15-30 minutes and dose again to bring it up to shock levels until it sticks to make sure things are taken care of.