I only saw this bug on the live server, and haven't tested it on PBE yet.

Morgana casts shield, and I wait til it's almost done and cast exhaust on her. After her shield ends, her move speed remains the same even while exhaust is still on her.

Edit: exhaust was applied before her shield ended, and lasted well after her shield disappeared.

This was probably intended. Her shield blocks all forms of cc while up no matter the duration, even if there is a persistent effect. i.e. Sion attempts to stun morg right before her shield breaks, even if the damage breaks the shield, she's not stunned.