Jetpack will always remove flexbox declarations?

Nevermind. Found the answer elsewhere. Since I can’t see how to delete this thread myself, I’ll just mark as solved and hope someone else can do it.

For others who may find this thread later, we’d like to add flexbox support to Jetpack’s Custom CSS module. You can follow our progress here:
https://github.com/Automattic/jetpack/issues/2297