This seems like a core issue, possible with 4.7, but I cannot find under https://lab.civicrm.org/groups/dev/-/issues
Once a Financial Account has been marked as deductible, it cannot be changed back to non-deductible.
To reproduce: Edit a non deductible Financial Account Check box for "deductible?" Save On the Financial Accounts screen, the account will now show as Deductible Edit the same Financial Account (which is now deductible) Uncheck box for "deductible?" Save On the Financial Accounts screen, the account will not have changed