I ran a L5 non-bountiful delve and used a L8 delve chest voucher/ticket/whatever, Nightshade Sanctum, and there was no bonus chest. Why?
The bonus chest that give good gear are in bountiful delves. I am unsure how you were using a L8 item in a L5, especially when there would be no chest to use the key on since it was not a bountiful delve.
Maybe you’re just confused that’s okay too, but only bountiful delves have the bonus chest at the end and you use a key to unlock them.
The treasure map thingies only reward gear in the same level (or higher) delve it was ran in.
Since you earned it in a tier 8, it will give a tier 8 level reward for your next tier 8+ delve ran.