Assuming I am following you:
The combining of funds with different expirations makes the whole amount take on the soonest expiration.
Unfortunate and kinda underhanded, I agree.
But this is nothing new. It is right on Southwest’s website:
“Southwest LUV Vouchers must be redeemed and all travel completed by the expiration date indicated. If applied in conjunction with any other accepted form(s) of payment, the earliest expiration date of all payments applies.“