Dragonslayer leveling bug

[Ancientlight]

Playing Elf
During raids from a fortified camp in several different map areas.
I have expended several experience points on raising the Dragonslayer's resistance to projectiles to 75%. However each time the experience point is used the value goes up to 75%, then a few minutes later I notice it is showing 100% and then it reverts back to 70% (I don't get back the experience point expended). I have tried this several times and it keeps happening. I think this first happened before the latest patch.
Hope this is sufficient information to help.

Can I get back the points expended on it?

Thanks Ancientlight