Other commenters have covered the workload lock-in angle pretty well. Using Kubernetes as a target platform for your application already gives you a decent shot a workload portability. Keep in mind though that some K8s APIs are leaky abstractions. You pay with lock in into K8s of course. At the end of the day, lock-in is a matter of tradeoffs.

An often overlooked angle is the "organizational lock-in" to the cloud. Adopting the cloud in any serious capacity with more than a handful of teams/applications means that you will eventually have to build up some basic organizational capabilities like setting up resource hierarchy (e.g. an AWS Organization with multiple accounts), an account provisioning process, federated authentication, chargeback... See https://cloudfoundation.org/maturity-model/ for an overview of these topics.

To be honest I have seen quite a few enterprise organizations that went through so much organizational pain integrating their first cloud provider that implementing a second provider is not really that exciting anymore. Now of course, if you plan on eventually leveraging multi-cloud anyway you can save yourself a lot of pain by setting things up with multi-cloud in mind from day one.

A good read on the topic is "Cloud Strategy" from Gregor Hohpe https://architectelevator.com/book/cloudstrategy/