Dynamics GP Sales Tax Qualifiers

We recently noticed that sales tax for local Florida county jurisdictions stopped calculating correctly.  While the proper functioning of this preceded my hiring, I'm assured at one time this worked.

What is required in certain FL counties there is a maximum taxable sale amount for a transaction of $5,000.  This means if a total sale exceeds $5,000 the tax only applies to the first $5,000, and the rest is not taxable.  We have the maximum taxable amount set up at $5,000 for the county tax detail, as required for this to work.

Well this apparently stopped calculating correctly in GP at some point, definitely before 2017.  Yes, at one time it did work, now it doesn't.  GP is calculating the tax on the total amount and ignoring the $5,000 max for the Tax Detail ID.

The source of this issue is the "Qualifiers" we have set to "Unit Amount" on each of these Tax Details.  Instead this should be set to "Invoice Total".  (See GP help for definition of the Qualifiers.)

Regrettably the UI does not allow changing this field once set and tax applied.  So it is necessary to override the value in the tax table using T-SQL to change it from Unit Amount to Invoice Total.

Once changed the tax resumed the proper calculation.

We are not sure when this "Qualifiers" field showed up in GP but it must have been after we created our tax details.  We've been using Dynamics GP since 2000.