AMPScript IF ELSE statement returns 'FALSE' unexpectedly

Question

I have the following AMPScript:

%%[
var @Custom_2, @variable

set @Custom_2 = Custom_2

if Not Empty(@Custom_2) then
     set @variable = concat(" ",@Custom_2)
elseif Empty(@Custom_2) then
    set @variable = ""
endif
]%%

And am printing the results in an email with:

%%=v(@variable)=%%

Custom_2 is a nullable text field. When previewing a record with an empty Custom_2, the word 'FALSE' is printed when the expected value is nothing should be printed.

Using this AMPscript below resolves the issue, as does renaming the Custom_2 field to anything else, but I don't understand why.

 if Not Empty(@Custom_2) AND Not(@Custom_2 == "False") then 
   set @variable = concat(" ",@Custom_2)
 endif

Both Custom_1 and Custom_2 share the same field properties. I've tried deleting the field from the data extension then re-adding it, only to encounter the same issue.

Is there documentation that explains why this would be occurring?