Flat top barrels should count as opaque blocks and therefore allow strong redstone power to be transferred through them, this is inconsistent with vanilla barrels

pictured here, a vanilla barrel at the top, redstone power is transferred through it

flat top barrel in the middle, power does not transfer through, this is inconsistent and not the expected behavior of a full solid block

normal limited barrel at the bottom, this one is slightly more excusable however, I would expect it to allow power to go through

Fixed this in 1.20 and 1.21 so that flat barrels can pass through the signal. Given that vanilla doesn't have any non opaque / non full block that would pass signal I am leaving it so that the default barrels don't pass the signal.