When I was living in Puerto Rico for work, the local credit union I was using had this same problem. Although the tooltip and messaging on the page said 8-16 chars, only the first 8 were used, and from my testing it had to be case insensitive.

I promptly updated my direct deposit with my employer and used my more secure off-island bank as the destination for the majority of my pay, and had only the minimum required to avoid fees and act as spending money put in that acct.