When I achieve level 3 of the reward track in the Hearthstone,I got two 2 Signature Tranquil Treant,then it asked me if I want replace my Tranquil Treant golden version to random cards,I clicked yes but it just give me two ordinary version cards.Shouldn’t golden cards be exchanged for golden cards?
Yes they should but its not a bug sadly.
It’s your non golden Tranquil Treant that got rerolled when you received the golden one
Set cards from the expansion reward track are not craftable and cannot be rolled, that’s why you got prompted to reroll the plain version of the card
He specifically said “it asked me if I want replace my Tranquil Treant golden version”.
Which I understand that they got confused by what got prompted to them, as first the received golden card will show up, then you’re prompted to reroll the plain one which obviously gives another plain one
You are never prompted to reroll golden cards received from expansion tracks
I did not get golden version Tranquil Treant from the expansion reward track ,it is signature version,and i got two golden version Tranquil Treant from the packs and they can be crafted.I’m not very familiar with the reroll rules.Can gold cards only be exchanged for regular ones？ 
My bad I didn’t check what was available in the paid track and assumed it was only golden cards
Indeed looks like a bug, you shouldn’t be able to reroll into lower quality cards, it defeats the entire purpose of the reroll