The strange thing with the AX370 Gaming 5 though is that slot 1 is the furthest away from the CPU socket for the first channel and slot 2 is the furthest away for the second channel. So that 'closest to CPU' logic is not in play here.
OP, please note the diagram in your manual. From the right hand side of the board, you should use the first slot, then skip one, then use the next slot. This is because the slots (from the right) are arranges 1/3/2/4. This is because slots 1 & 3 are on channel 1, and slots 2 & 4 are on channel 2. To get dual channel, you want one stick on each channel. Hence the odd numbering system.
Why you should avoid slots 3 and 4 for a two stick layout though, I have no clue considering they are actually closer to the CPU socket. Every mobo I've seen does have such a recommendation though (but usually those closest to the CPU socket, unlike this one).