Which version? There's an issue in 12c that I know of but I'm surprised you mentioning that being one in 11...
the version is 188.8.131.52.151020.
You may try this, although the solution changes start page for all users: https://www.linkedin.com/pulse/change-home-link-other-dashboard-obiee-11g-priyajit-mishra
and perhaps this may be useful also: https://community.oracle.com/message/13081666#13081666
I tested the scenario you mentioned in 2 different obiee versions - 184.108.40.206.0 & 220.127.116.11.160119. Both version redirects me to the dashboard page specified in My Account on login.
If a user has set soemthing in his "My Account" profile then that will win. Always.
Hierarchy is like this: Homepage (install default) loses to PORTALPATH which loses to My Account.