Thanks for getting back with us with the solution, as QandA platform doesn't allow users to accept their own answer, ill post the same solution you gave here, and kindly accept it, as it helps people with the similar issue.
Solution : Turns out there was a process running that updated a plot when a plot sale is updated, even though the plot foreign key wasn't required. Turning off this process has fixed the issue.
Thanks much for your contributrion.