If I use Spiritomb's "Ticking Terror" attack against the Defending Pokemon and their owner moves it to the Bench during their next turn, does its Weakness return to normal?
Yes, since it is an attack effect, moving that Pokemon to the Bench removes it.
Source: Brilliant Stars FAQ; TPCi Rules Team (2022-02-24)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH9_89
SWSH9_89