Soul Crystal 10 - have some use limits?

[Ambicija]

Weapon r95 was have 2 Soul Crystal - mermoden 9lv (m.atack) and mermoden 7lv (m.crit dmg).

So i got 1 soul cristal mermoden 10lv -  remove old mermoden 7lv - and can't upgrade with mermoden 10lvl on weapon... then i remove next mermoden 9lv - and get option to use mermoden 10lv

So im on clean weapon put mermoden 10lv (crit dmg) and its ok. But now cant put 1 more mermoden 7,8,9,10 lvl (m.atak).

Question - its bug? Some limitation? Or my mistake?