mp4tags sets wrong value and mp4info expects wrong value for explicit rating

[Phernost]

mp4tags -X explicit somefile.m4a

Sets the rtng atom to 4, not 1 as it should be. Similarly using mp4info on a file with the correct rtng of 1 gives:

Content Rating: UNDEFINED(1)

This should obviously be marked as explicit. The confusion can come from how iTunes displays rtng. Missing the rtng atom or having a value of 0 are treated the same. A value of 2 displays the [clean] graphic next to the track title in track listing, but not in the player status, Info Editor dialog, or in the iOS music player.

Any value greater than 2 displays the [explicit] graphic in that same manner as the [clean] graphic, but only a value of 1 also displays the red [E] in the player status, the Info Editor dialog, and in the iOS music player.

Therefore while a value of 4 may display as explicit in some places, it is not treated as explicit in that same way as a value of 1 is. In fact any value other than 1 will allow iTunes and iOS to play the track when Restrict Explicit is enabled.

As such the fact that iTunes displays values greater than 2 as explicit, and the fact that mp4v2 assumes a value of 4 is explicit, is a bug.

Ubuntu 16.04.1 LTS
mp4v2-utils 2.0.0~dfsg0-4