The Mansfield toilet that you are removing, was never very good.
They were "builder grade" toilets, meant to complete the home for sale.
There are entire communities built with these, and they provide a constant source of income for someone like me, removing and disposing, and installing "homeowner grade" toilets that function and work.
Some areas like Bellevue, Issaquah and Redmond are providing rebates to encourage replacement.
You may have something similar in your area too.
A toilet has a small trapway, with bends and places that catch.
Odds are, what ever was causing problems, is still there stuck in the lower portion of the toilet.
Once it passes through, it drops into a 3" pipe with nice rounding bends.
It's very unlikely that anything stays in those pipes, unless you have roots growing into a joint, or a pipe collapse.
If that were the case, it would be backing up everywhere, not just the toilet.
Consumer reports likes the Mansfield. I don't.
Way too many clogs with a Mansfield.