Corporate Tax not Working Properly

I had a 20% corporate tax rate and was making about 4-5B/month on it. When I dropped it to 10%, as soon as it hit 14$ (down 3%/month), it dropped to literally $0, and now its stabilized at about 100-150M/month. How does a 50% tax drop (20% to 10%) result in a 96-97% drop in tax collection? Also my state corporations are still steadily paying me 4-6B/month as profit, so I know that's not the issue.

Taxes are only paid on profits. It is possible the corps weren't as profitable on the next month.

Their profits were little changed, and now my corporate tax has inched up every month and is now about half of where it was (where it should be since I dropped it from 20 to 10). This is definitely not normal behavior.