There is often confusion about which order overrides and the various append operators take effect.

OVERRIDES = "foo"
A_foo_append = "X"

In this case, X is unconditionally appended to the variable A_foo. Since foo is an override, A_foo would then replace A.

OVERRIDES = "foo"
A = "X"
A_append_foo = "Y"

In this case, only when foo is in OVERRIDES, Y is appended to the variable A so the value of A would become XY (NB: no spaces are appended).

OVERRIDES = "foo"
A_foo_append = "X"
A_foo_append += "Y"

This behaves as per the first case above, but the value of A would be "X Y" instead of just "X".

A = "1"
A_append = "2"
A_append = "3"
A += "4"
A .= "5"

Would ultimately result in A taking the value "1 4523" since the _append operator executes at the same time as the expansion of other overrides.

Key expansion happens at the data store finalisation time just before overrides are expanded.

A${B} = "X"
B = "2"
A2 = "Y"

So in this case A2 would take the value of "X".