I think Fallback plan is implemented when the contingency plan fails or is not fully effective. In other words, you can say that the fallback plan is generally made for residual risks. It is a backup plan for the contingency plan.
The fallback plan is also a part of the project management plan and defines the cases where actions have to be taken.
Workaround is used when an unexpected issue occurs which is not identified until it occurs.Its like an emergency mode.
In my understanding the key is whether the planning is done before the risk occurs or after
A Fallback plan is prepared in advance for a situation, where the primary plan needs to be abandoned, so it is planned before, but as a secondary plan
A workaround is not planned in advance, but developed when the risk has already occurred or the planned response has not worked.
Training for Project Management Professional (PMP)®, PMI Agile Certified Practitioner (PMI-ACP)®, and Certified Associate in Project Management (CAPM)®