I get the same problem Sauteed_Onion reports here https://forums.rpgmakerweb.com/index.php?threads/yanfly-row-formation-positioning-help.95774/, but it is not created for the same reason (ie I don't add any new character to the party during combat).
I'm using Yanfly's Row Formation plugin http://yanfly.moe/2016/01/02/yep-54-row-formation/, with two rows, with the default game resolution.
By default, two of my actors are on row 1, and the two others on row 2. Everything is fine in this situation, as well as when all four actors are on the same row, or when three of them are on row 2 and only one on row 1. The problem occurs when three actors are on row 1 and one on row 2; then, the actor on row 2 is right in the back of one of the other actors and comes into him/her when advancing to choose an action for the turn. I put a set on screenshots to illustrate all this under the spoiler tab:
Spoiler (images)
I tried playing with the plugin's setting, but to no avail; I can't understand what each setting really stands for, so I'm having a hard time figuring out what to do. What can I do to fix this? Thanks!