summary: this line should be in_array($taxDetail->getTaxSubTypeId(), $landedCostSubtypes, true)
when $taxDetail->getTaxSubTypeId() returns null (as seems to be the case for most of my GetTax calls) and the config for fixed types is empty string making $landedCostSubtypes have a value of [''] (the default) this line reduces to
return in_array(null, [''])
which is TRUE in php when you leave the default $strict param as false.
https://github.com/onepica/avatax/blob/baac2b0515708bfab3f7abe0406648bd9f3942ea/app/code/community/OnePica/AvaTax/Helper/FixedTax.php#L59
summary: this line should be
in_array($taxDetail->getTaxSubTypeId(), $landedCostSubtypes, true)when
$taxDetail->getTaxSubTypeId()returnsnull(as seems to be the case for most of myGetTaxcalls) and the config for fixed types is empty string making$landedCostSubtypeshave a value of[''](the default) this line reduces toreturn in_array(null, [''])which is
TRUEin php when you leave the default$strictparam asfalse.