I bought a full set of ilvl180 gathering gear and had the LaNoscea materia melder meld for me Perception Materia IV in both slots of each of the pieces I bought.

Unexplicably, the Gaganaskin Gloves of Gathering second slot said the item level was not high enough and it would be reducing the materia effectiveness, so I proceeded to put in a Materia II instead, and I still got the same issue.

Even worse with the Bloodhempen Trousers of Gathering (which, btw, is a naming bug since everything else in the set is named Gaganaskin), since even the first materia slot could not accept the full power of the Materia IV.
Another difference of this piece of gear with the rest of the set is that just under the materia melding there's a tailor icon followed by number 53, while all the other items (gloves included) have a leatherworker icon and number 54.

In short, for what concerns materia melding, those two items are currently set, internally, to a lower level than they should be.

Date & Time: Anytime
Frequency: Always
World name: Omega
Character name: Shard Baneshyr (equip my fishing gear set)
Class/Level: Fisher lvl 61 (but it's not class/level specific, it's gear specific)
Steps: Detailed explaination at the beginning of the post. Just try to un meld and re meld materia IV in the two objects.