This is unusual behavior and hard to tell exactly the issue without the FirePass version but might have found a few solutions. First, F5 does have some resources that might help:
AskF5 Knowledge Base: http://www.f5.com/solutions/resources/knowledge-base/
F5 Support: 1-888-882-7535 or (206) 272-6500
and came across a few solutions from our AskF5 Knowledge Base (might require register/log-in):
SOL9431: Users cannot change their Windows domain passwords on the Tools: Account Details page: https://support.f5.com/kb/en-us/solutions/public/9000/400/sol9431.html
SOL11195: The Windows cached credentials are not updated when the Active Directory password is changed: https://support.f5.com/kb/en-us/solutions/public/11000/100/sol11195.html
SOL4114: FirePass may not allow Active Directory users to change their passwords: https://support.f5.com/kb/en-us/solutions/public/4000/100/sol4114.html
Hope that helps and if not, please contact support.