debuffs are covered by buffs, that pendant icon just happens to be his only buff at the time.

Confirmed — and John's follow-up is exactly right, it's not the pendant specifically, it's any buff.

What's happening: the debuff row works out where to sit by assuming the buff icons above it are 42 pixels tall starting 14 pixels down. That was true once, but the buff tray was since rebuilt to the original client's real numbers — 32-pixel icons starting 55 pixels down, hugging the right edge with a 3-pixel margin. The debuff row never got updated to match, so it computes a position that lands right on top of the buff icons instead of below them. The buffs are also drawn on a higher layer, which is why the buff wins and the debuff is the one that gets hidden.

Fix: have the debuff row read its position from the buff tray's actual numbers instead of the stale hard-coded ones, and use the same cell size and right margin so the two rows line up in a neat column. Then a debuff always sits cleanly under the last buff, whatever you have running.

Good catch — the debuff badges were literally underneath the buff icons. The debuff row is meant to sit as a second row below the buff tray, but it was still positioned using the tray's old layout (42px icons starting at y=14). The buff tray was since moved to the authentic v83 UIBuffList spot (32px icons starting at y=55), so the debuff row was being drawn at y≈64 — right on top of the buff icons — and because the buff tray draws at a higher depth, the Pendant of the Spirit icon covered it. The row now starts below the buff icons and their countdown labels, so both are visible at once.