It's weird, I used Enduring Melody with Fantastic Chorus, and I was looking at my Character Window...
My Defense was slowly decreasing!
It went into the red eventually!
It decreased any time it passed a Tick in the skill or whenever I changed my equips!
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Edit: Hmm... maybe it was catering?
I made a dish of 10x Tempura Udon and it really boosted my defense/protection
I CC'd and the negative effects cleared up (I was down to 0 defense!)
I used Enduring + Fantastic and nothing weird happened. Spamming Chaning equips didn't do anything out of the normal.
I laid down another catering dish with +def/prot, and then used Enduring/Fantastic again, and the glitch happened again! Spamming equip change caused my defense to lower over and over until it reached 0 again!