it's not only amulet, it's all items that give stat bonuses.
I noticed, that this bug reproduces on companions in locations, where you have met them.
just put any item with stat bonus on your companion , save -> load (in specific location: for Aloth, Eder - Gilded Vale; Durance - Margan's Fork), you will get bonus permanent (
I have the same issue.
Save with broken stats
https://www.dropbox.com/s/417py8z409uc9wa/0f49328c-d8c9-4cbd-97dd-0a429727c91d%20quicksave.savegame?dl=0
It worked fine couple hours ago, and i can't reproduce the bug on earlier saves