is the error message accurate in that your file has eight channels or planes or layers? or maybe it has fewer, but an unusual data type?
I would have expected IMREAD_UNCHANGED to return the data as is… might indeed be a bug.
if this hasn’t been reported by anyone yet, you should submit an issue on the github.
I browsed the code a little. It’s an assertion without any immediate reason.
Maybe this can be raised with no ill effect.