There are TWO DIFFERENT "Master-crafted Leather Equipment" sets -- one RARE and one UNCOMMON.
(Both contain might-based Medium Armour, added in Update 15, obviously for the Beornings.)

So far, the following quests have been identified as awarding one set or the other.

It would appear that the Uncommon set is in error, as the Rare set is awarded for lower level quests.
Both sets have identical "stat bonuses," however, the Uncommon set has LOWER Armour values than the Rare set.
Both sets are Item Level 30.

:* [26] [[Quest: Seeking the Source| Seeking the Source]] - This is the '''Rare''' set
:* [26] [[Quest: Ending the Elder| Ending the Elder]] - This is the '''Rare''' set
:* [28] [[Quest: The Troll-keeper| The Troll-keeper]] - This is the'''Rare''' set
:* [28] [[Quest: Disarming the Valley| Disarming the Valley]] - This is the '''Uncommon''' set
:* [29] [[Quest: Scouting the Ram Dúath| Scouting the Ram Dúath]] - This is the '''Uncommon''' set
:* [31] [[Quest: Cutting off the Hand| Cutting off the Hand]] - This is the '''Rare''' set
:* [31] [[Quest: Dead Water| Dead Water]] - This is the '''Uncommon''' set

(Ignore the WIKI-markup in the above list.) For details see:
https://lotro-wiki.com/index.php/Item:Master-crafted_Leather_Equipment_(unc ommon)#Quest_Information
and
https://lotro-wiki.com/index.php/Item:Master-crafted_Leather_Equipment_(rar e)#Quest_Information

Yes, this has been submitted as a bug.
And yes, this is a duplicate post from the general forum thread (Beorning Feedback), as I just discovered that this Beorning thread existed....