is Mithral full plate armor a loop hole?

We are playing a one-shot at Level 7 and I allowed the players to choose 1 magic item of rarity uncommon and minor tier for their character. Also they can have items worth 300 Gold in addition to starting items.

One of the players wrote me whether it is a loophole to choose a Mithral full plate Armor (DMG 182) as magic item. A full plate armor costs 1.500 Gold and is therefore way out of reach for the starting Gold, however as mithral full plate armor it is a uncommon minor (XGE 141) magic item. Since it is just a one-shot I’m not so concerned about it, but in general I think this might be a problem. For example in a campaign such a rule could allow new characters to appear with full plate at level 7. So my question is:

Is Mithral full plate armor a loophole to abuse such starting magic items or is it balanced to get this item at level 7?