If it's working as intended, then the patch notes you quoted are incredibly misleading. We're not talking about exceeding the 300 point maximum. When I was at 90 points combined, I was occasionally losing 0.01 points of one skill when I raised another. Those patch notes say that the skill level of other classes will only go down in certain instances, and then it explains those instances by saying it will happen when you try to go above the 300 point limit.
If it's intended that every skillup could result in a skilldown, the patch notes should have said:
Successfully desynthesizing an item will raise your skill level for that class. However, this may also lower the skill level of other classes. In addition, if the skill level increase would bring your combined skill level over the 300 point maximum, the skill level of other classes will always be lowered to bring the combined skill level to the 300 point maximum.