Using /player <yourname> shadow resets piglin aggro.
In earlier versions (only tested in 1.16.5) this worked fine. - I don't have enough time to re-check this. I believe that his was the case, but I wouldn't bet on it.
Could be related to issue #1190 .

CommandLeo pointed me to a possible explanation by Ghoulboy.
Shadowing is technically relogging and that's enough for piglin to lose aggro.

Anyway, this is most likely not intended behaviour, or at least I would argue for a setting to change this.

1.18.1 Fabric server
Carpet v1.4.56
Fabric API v0.44.0
Lithium v0.7.6

To add to this, at least in 1.19.1 with Carpet 1.4.83 I can't get Piglins to aggro on a shadow-bot at all. Before shadowing myself I put snowballs in my inventory and made sure to look at an angle where throwing them had a chance to hit Piglins. After shadowing, I then logged into the world with my camera account and used that to issue /player use commands to my shadow-bot. As expected, it threw snowballs, and I did get lucky enough to hit to or three of them – they take damage, and the ones I hit directly are indeed aggroing and run to their death in the farm, but the aggro doesn't spread like it does when I do the same thing directly as a normal player.