You would expect very little possibility to see the coordinative や and the case particle に next to each other.

や factors out the shortest different sections, so you will find [particle]+や in a XやY composition that X and Y ends in different particles.

{東京や仙台]から来た人 (diff. nouns w/ same particle)

 

在籍期間が[2012年まで‌や2018年から]の人 (diff. nouns w/ diff. particles)

That means the theoretical order would be にや, but in reality, you will be not likely to encounter such a situation, because に is one of central case marker in Japanese, and や stricter about syntactic leanness than its kinds* (such as か, とか and やら), which means, you need another particle, that is different than に but compared to に, to make something like にや appear. That would result in a very terrible type of zeugma, like conjoining subject and object together.

* cf.

じぶんがいじめられたら先生に‌とかともだちにそうだんする。
× 先生に‌やともだちにそうだんする。

You would expect very little possibility to see the coordinative や and the case particle に next to each other.

や factors out the shortest different sections, so you will find [particle]+や in a XやY composition that X and Y ends in different particles.

{東京や仙台]から来た人 (diff. nouns w/ same particle)

在籍期間が[2012年まで‌や2018年から]の人 (diff. nouns w/ diff. particles)

That means the theoretical order would be にや, but in reality, you will be not likely to encounter such a situation, because に is one of central case marker in Japanese, and や stricter about syntactic leanness than its kinds* (such as か, とか and やら), which means, you need another particle, that is different than に but compared to に, to make something like にや appear. That would result in a very terrible type of zeugma, like conjoining subject and object together.

* cf.

じぶんがいじめられたら先生に‌とかともだちにそうだんする。
× 先生に‌やともだちにそうだんする。