The Senate is not in recess; therefore, these appointments are blatantly unconstitutional.
This is a procedural move by Congress. I read an explanation of this, and from what I understand, it’s a technique that was first used by the Dems to block a GOP president’s appointment (that is, Congress was not technically in recess, but at the same time, they weren’t actually in session, either). It was upheld by the courts as Congress being in session, thereby preventing the GOP nomination. I don’t know if it ever made it to the Supreme Court, but it was upheld at lower federal levels when challenged by the GOP. However, the GOP was then able to use it to prevent some Clinton “recess” appointees.
The problem is that now it would not be upheld by any court in the land because it would have a negative impact on a Dem president, and we all know that this cannot be permitted.