Some more info on this. This bug only seems to appear if you first add a charge counter to the relic on the turn you cast it. Then you will get the extra mana as expected on the following turn, but also get an extra mana on each subsequent turn regardless of whether or not you put a charge counter on it.
But if you instead do not put a charge counter on it the turn it enters play (and just tap it for mana instead), it correctly doesn’t give you the extra mana on the subsequent turn.