My guess is that the memory manufacturers have stopped producing 12Gbps chips in favour of the 14Gbps ones, as it simplifies logistics and inventory. (This is the same reason NVIDIA axed TU117 in favour of using die-harvested TU116.) The fact that all the 1650 Super cards reviewed so far use 14Gbps memory supports this. But NVIDIA can't allow 1650S cards to ship as 14Gbps because then the only difference between those cards and the 1660S would be the number of CUDA cores enabled on the GPU (1280 vs 1408) and the amount of memory and bandwidth (4GB vs 6GB/192GB/s vs 336GB/s), which would cannibalise 1660S sales. So, 1650S cards get a BIOS limiting their memory to 12Gbps despite the fact that they can all do 14Gbps no problem, in order to keep the marketing segmentation.
The end result is that the 1650S is probably the bang-for-buck card in this market segment, assuming you overclock the memory to get the ~10% free extra performance that nets you. Unless the mass-market RX 5500 is significantly faster than the OEM version W1zz reviewed, or comes in at $140 or less, it's going to have a hard fight against the 1650S.