Weird simulation interactions when the frozen floor body does not have the mass property set

While developing the swimmer env, I realized that the mass property of the floor body is really important, even though it is frozen and thus is acting like something with infinite mass. First of all, I found the first body of the swimmer to be slightly overlapping with the floor, then later as I applied a random action to see if the swimmer works correctly, its body parts moved around randomly. Adding ```mass: 1``` resolves the issue completely.

So my question is: Does the mass property in frozen: all bodies make sense somehow and if not set, should it be set to a default value to avoid simulation issues?