"Some how my Witness server, (2-node cluster with Witness), is using an ESXi evaluation license."
There are 2 main ways this can occur both of which are usually caused by the person configuring this clicking Next>Next>Finish without adequately reviewing what is being applied:
1. User-error selecting the Evaluation license on deployment instead of the embedded license,
2. Using a version of client that isn't aware of what a Witness is and thus doesn't see its embedded license (e.g. C# Client or earlier versions of the vSphere Client) and thus only shows Evaluation/full license as available options which the user then applies:
Currently the only supported way of remediating this is replacing the Witness appliance (and choose the embedded license correctly this time).
While there are potentially other conceivable ways of remediating this (e.g. finding a way to get a valid Witness-embedded key and borrowing it), tricking vCenter into adding the Witness back as a 'new' Witness to vCenter inventory etc., I have had extremely mixed results with this (e.g. sometimes it works, sometimes it doesn't, sometimes it works and seemingly breaks other more important things) and thus advise following the supported way of remediating this.