So for the system "Wing Etherealization(under character page)" you can acquire skills if the left and right wing are the same type(int/int str/str/ mp/mp, e.t.c.)
Furthermore, when you upgrade them via reiki stones, once the right and left wing reach a certain level, the skills will upgrade. For example:
when the left and right wing reaches - you will receive:
1000 - lv 2 skills
3000 - level 3 skills
5000 - level 4 skills
10,000(max) - level 5 skills.

The problem lies that when you equip wings again and put the skill in your hot keys, it's always level 1.
So I think it may be a bug that the skills are only level 1 no matter how much you upgrade wings despite the interface showing that they can level.
Note: even if you drag the skill from the interface itself, it'll say whatever level the skill is but when you equip wings, it'll revert back to level 1.

For picture proof it may take awhile since rebirth recently and the system can only be used at 80+

In the meantime I can only show this:
1) First picture shows that I have level 5 reiki Veins Awakened
2) Second picture shows that on my hotkey it's only level 1/5
Attached Files