Need help with bromine level because I’m a novice

[OasisMinder]

Brand new 350 gallon unit. Filled the tub for the first time 3 days ago. Balanced PH and the TA was spot on.  I used a granular bromine starter and put in the suggested amount for the tub size (unless I confused measurements). Anyway, the bromine reading was off the charts high and I had a bunch of kids egging me to hop in. So, after researching out ways to lower, I dumped maybe 50-70 gallons and replaced with new water.  This lowered the reading to the correct level on the tester and the kids got a good 20 minutes in the water at the end of the night.

...the next morning, the bromine reading was zero. I then put 3 brom tabs in a floater.  Yesterday morning I shocked the tub as well. All day, still a zero reading.

...this morning, still at zero. I really don’t understand. Did I mess up the bromine bank when I dumped 70 gall and replaced with hose? Do I need to dump the whole thing and start again, or can I right this ship? I’m still fairly 🤷🏼‍♂️ When it comes to hot tub chem.

(And Waterbear’s post has helped me understand a lot more since I stumbled...)

thanks...