There is a context menu for reversing a motion clip in the timeline.
However, there is no such option for expression tracks.

Expression clips should also have the option to reverse them.

Here is a thread where some user asked about that feature:

https://forum.reallusion.com/410476/any-way-to-reverse-a-Face-Puppet-clip?Keywords=reverse

At least a few users liked that post, including myself.
So, there is some demand for that feature.

I see no reason why motion clips can be reversed, but expression clips not.
Please implement that feature.