The Runemaster weapon, for example, has up to 270% Ed (native Ed%, already has Ed% when dropped) and 5 slots. I discovered that the effects of jewels dropped from ancient colossal objects are cumulative, but in the case of the Protector's Stone, it has an additional Ed% in its attributes. Even if you use a magic weapon that has Ed% (native Ed%, already has Ed% when dropped), it would cause a bug in the weapon. The other attributes of the jewel would be cumulative, but the Ed% would cause the bug.
No bug on weapons
Also no bug on non weapons with native %ed
I've tested / confirmed