The nf_whip_notice() is incorrectly using transient instead of normal option.
Created by: Luke-SF
I see in the latest versions of Ninja-forms the nf_whip_notice for old PHP versions is checking for a transient. Transients may or may not expire at the time limit specified, or at any point before that depending on what type of cache system the site's using. See https://wordpress.stackexchange.com/questions/226200/why-are-transients-cleared-prematurely for details.