Desynth Issue

[LyrioDC]

If you read the patch notes, it warns you that leveling another desynth craft can and will lower your other ranked desyth crafts.

[Maluber]

What the OP is trying to say is that his CUL skill is decreasing while being within the 300 limit.

The patch notes are as below and nowhere does it suggest you will be losing skill while leveling other desynth crafts.

Successfully desynthesizing an item will raise your skill level for that class. However, this may also lower the skill level of other classes in certain instances.
* If the skill level increase would bring your combined skill level over the 300 point maximum, the skill level of other classes will be lowered.

Edit:
@Dark are you playing on the ps3/4 as patch notes do say and could be worth a look into.

* An issue has been discovered when desynthesizing on the PlayStation®3 version. Please refer to the Known Issues section for further details.