Redcloak missed one

[Kish]

you don't think something like "on the morning of so and so we heard the phylactery had been recovered" might not cause xykon to wonder why he didn't hear of it till that night?

It might, but the time involved was clearly nowhere near that long.

Tsukiko came to Redcloak's study (...just before Redcloak got back, and well before he started casting spells on the fake phylactery...) already knowing the phylactery had been discovered, and its discovery reported to Xykon. Redcloak went straight to his study with the phylactery, and confronted Tsukiko there. After killing her, he started casting spells on the fake phylactery as he walked to bring it to Xykon. Presumably he cast a few spells off-panel, between saying the word "Greater--" and reaching the creature in the darkness, but unless you're suggesting that from Redcloak's study to the creature in the darkness' box is a walk that takes multiple hours...and even if you are, Xykon would know that there were lost hours between the phylactery being found, stolen, and recovered. (And if there was any error in concealing this hypothetical time-skip, it would be in saying that he killed Tsukiko ten minutes ago, if Tsukiko actually disappeared hours ago.)

(I also wonder how long you're thinking Jirix would have survived Xykon's rage.)