Ok, since you are saying that all those things that I said are a given, then let's leave those out, clearly this won't help your current problem.
Guard works identical on all jobs and has been changed from 7.08 to 7.1 to the one you are experiencing on DRK. You are welcome to send me video footage on Discord (@doehardt) to show me otherwise, perhaps there is a detail that I am missing to read or you are missing to explain.
But to simply explain - Guard and essentially ALL BUFFS AND DEBUFFS have changed netcode wise from "you press button and it snapshots from button press" to "what you see is what you get". If a buff/debuff is visible, it is active. If the buff/debuff disappears, it's inactive/gone. This means as soon as you drop Guard for ANY REASON and the buff disappears from your HUD, your mitigation is gone - no backsies, no server ticks saving you like in PVE. Inversely, you can sneak in Guard/other buffs and debuffs before damage lands as animations are playing.
The reason why you find this wrong is because you have been playing with snapshot-based FFXIV/PvP for the entire time. The days of being able to safely execute a Quietus out of Guard is gone, simple as that. No job has this luxury anymore and they traded it for being able to use Guard when attacks are in animation and mitigating that instead. Bonus info - your position when using Impalement also matters, as the position snapshot is equally at the end of the animation. This means you can wind-up the hit while nobody is in reach yet and move into enemies to line up the AOE attack.
The point is, Guard works, just not in the way you want it to work - it's not bugged. I am actively PvPing and DRK is my go-to job in Crystalline Conflict, so I can with great confidence tell you that it works as intended with the new snapshot system, assuming decent latency (<50ms).
PvP simply changed and certain techs won't work anymore while others will now. You can either accept that and get used to it or join the others in wanting a revert to the snapshot overhaul for all of PvP, because that is your actual problem with Guard.