t3lib_transferdata::renderRecord_flexProc accepts $fieldname but does not pass it to t3lib_BEfunc::getFlexFormDS
As topic says, the transferData method that processes a FlexForm field for a table already accepts $fieldName which contains the lowercase_underscored database field name that wants its FlexForm content processed. However, $fieldName is not passed along to t3lib_BEfunc::getFlexFormDS (most likely because the built-in processing never required $fieldName to be filled in order to just process the data) - but, after hooks have been added to FlexForm processing in t3lib_BEfunc::getFlexFormDS and these hooks also use $fieldName, we need to pass that $fieldName from transferData through to BEfunc when processing "flex" type fields.
Behavior before: hooks always called with empty fieldname regardless of which, or how many, fields for $table required processing. Multiple hook calls would occur and there would be no way to determine which call was related to which field.
Behavior after: $fieldName transferred as parameter to all hook calls; hook methods can now use that parameter.
Fix is made for 6.0 trunk should be backported as bugfix as far back as 4.5 LTS.