To be honest, it depends on whether it was a scheduled break for you or not. I have one part-time term in my undergrad career too. For me, Summer 2017 was a scheduled break and I took a part-time load. I am guessing you can take full/part/no load in your scheduled breaks. So, in this case, it should be okay. I still included a letter of explanation stating that Summer 2017 was a scheduled break.
No I was part-time in Fall 2019 (not my last term nor a scheduled break) but maintained full-time for all the other terms(including summer).Nobody can really say, you are only allowed a part time semester in the last semester. If there is a part time course then it should be explained in the LOE or in the webform.
I did include a LOE saying I didn't take other course before of the prerequisites, not sure if this would help.
Anyone got approved with part-time semesters not during a scheduled break?