Last marker disappears when you add another

I need to add multiple markers to a map for a client. I added 5 markers to my map, then saved it. I went back to add 5 more, saved, and realized that I only had 9 markers total. After some testing I realized that the last marker always gets removed when you add others. It seems to be a logic error when it comes to adding to the array of markers.

Thanks for looking into this.