I have to think it’s the type of LEDs you were processing, and/or you had some losses somewhere? For comparison below I’ll link a test video that 999 Dusan made about 4 years ago. He’s been refining a long time end I trust he knows what he’s doing. My short recap is:
He used a mix of different sizes and colors of old-style DIP package LED’s.
He started with only 179 grams (then removed any external leads still left with nitric acid, so even less weight?).
After incinerating and crushing, acid processing, etc, he recovered 1.2 grams of silver which you didn’t even mention in your post.
He also got a bead of gold which was less than .1 gram so his scale wouldn’t weigh it. Visually though it looks to me like it was probably more than your result of 28mg? But he started with only 1/3 the amount of material that you did?
For comparison he has another video he did a year later where he processed only the little square white surface-mount type LEDs. While he got a good amount of silver, they contained no gold at all.