The tripwire hook pairs, connected by string, will stay down after a MOB has triggered them and exited the bounding box of the string. This can be confirmed, and replicated, on the shifting floor designs used in farms such as ImpulseSV's ghast farm (a.k.a. The Scream Machine), or Snocrash's wither skeleton farm (a.k.a. Wither Skeleton Sandwich). I can't confirm with any degree of certainty that this bug occurs in other configurations.
It arises when MOBs fall through the shifting floor, and can take some time to appear, although there is no certain time that needs to pass for the bug to develop. It could occur after only a few seconds. Could it be due to the MOBs exiting the strings bounding box from the bottom of the bounding box, rather than the sides or the top? Perhaps the string object is missing the MOB exiting its bounding box from below and not generating an event message. That is what it feels like when I have observed this happening.
Moving to a location that unloads the chunks that the tripwire hooks are in, and then reentering those chunks results in the tripwire hooks being 'reset' to their correct, inactive, condition.
Below is a link to a video that I made to show this bug while it is occurring. I'm sorry about the amateur quality of the video, but I only made the video for this bug report.