For my part I ask you (sorry if you said this and I did not understand) what "custom" fact are you adding and what are you trying to accomplish with the fact?
In over 20 years of using various genealogy programs I have strategically moved away from custom facts and replace that with using the GEDCOM standard to its fullest.
FTM does not support GEDCOM well so FTM is not my primary repository for genealogy. But I do record many things that are not "Facts" but "Theory" and I don't use a cusom fact to support this, I insted use the QUAY and NOTES tags in the source_citation and source records.
I have plenty of occurances where something I thought was correct turned out to not be correct based on additional research. This in some cases occured 10 or 15 years after the original data was entered.