You may have solved it-- it may have been the test vial or the spinner.
I had to take the kids to the shoe store, which took longer than expected, so I wasn't able to do hourly tests for a bit, but...
Old vial, at 10:30 FC 18 CC 8, added 1 gallon of chlorine
New vial, At 2:00 FC 22 CC 0.5...