Hi,
now i´m relatively sure, that my first assumpion is true: Layout proble: Ground bounce.
--> You don´t have a GND plane! you may have some copper poured areas ..but still extremely bad GND wiring.
See the GND path in pink:
1) it´s a long way to go
2) there are several paths with extremely thin wires
3) the GND current on it´s way to the connector is "added" = influenced by the current of other channels. (current of Q16 ...+Q14 ....+Q15)
4) every voltage drop on a wire/trace is added to the voltage drop of the resistor --> causign wrong current readings
--> make the top side a solid GND plane.
If you want to make it with
individual traces (worse than GND plane!) then use
star connection to GND connector with
wide traces to avoid influence of one channel to the other.
Also use individual star connection for each amplifier connection.
***
On top right corner I did some improvements. With a little description.
You see the GND plane. Solid!
Klaus