I got the "voucher cannot be used for this checkout" message when I tapped the checkout button in the few minutes the 50% off voucher was available. That, in itself, is strange because typically if the voucher is not applicable, it would not be selectable in the first place. Also I had "tested" the item I wanted to purchase with the regular 10% off voucher, which has been a pretty good indicator of eligibility.
Unfortunately, after a few refreshes the voucher was "fully redeemed" and I couldn't test it with other items.
I'll try again with a different item tomorrow but I suspect it's just Shopee cutting off our supply lol.