When utilizing the 'Post' button on any form with activities filtered on it, users may notice that the button ignores any mandatory fields on the form when posting an update.
Release or Environment
This is expected behavior.
The post button is built into live feed and is not able to be modified. The purpose of it is to simply post quick updates to the incident (or whichever form you wish to comment on), and mandatory fields on the form itself are not evaluated again in this case, as this is the normal out-of-box behavior.