Sithyrr'tyne / SD sneak Attack bug?

[MoonlitRitual]

Description
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As part of its properties, the Sithyrr'tyne provides 5d6 sneak attack bonus, which should not collide with the Improved Sneak Attack provided by a shadow dancer's shadow summon. In effect, whenever the bonus Improved Sneak Attack is re-introduced ( i.e when you exit and enter the aura that grants it), the normal sneak attack is changed to 1d6 in your character sheet (also evident by a significant drop in damage), while at times It seemed I had lost the ability to perform a sneak attack altogether.

It should also be noted that I do not have a sneak attack as a part of my class selection - It comes solely from the weapon.


Steps to reproduce
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Enter, exit, and once again enter your Shadow's proximity to enjoy its Improved Sneak Attack bonus.

Expected result
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Retaining both the Sithyrr'tyne's 5d6 sneak attack and the separate Improved Sneak Attack.

Actual result
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While the improved sneak attack seem to remain, the 5d6 sneak attack either becomes 1d6, or is lost altogether (I had times when both a 1d6 and 5d6 feat were present in my sheet, yet mostly it is evident by losing the feat altogether).