When someone is given a new role (or has a role removed), the change does not affect the current session.
I tested this in two browsers, logged into one as an Administrator (Ethan) and another as a normal user (Fako). When Ethan gave Fako admin permissions, Fako reloaded his page was immediately able to see restricted comment. Then Ethan took his admin permissions away and the restricted content was immediately gone.
I also tried to reproduce the problem in automated tests but was unable to.
It's possible that some recent upgrades that have cleaned up cache clearing (particularly those moving actions to the new events api) have fixed some loophole by which some card's cache was not previously cleared.
The following are technical notes on role caching / clearing for future reference:
Unless someone is able to reproduce this bug in the next week or so, I would vote for closing this ticket.