Entity max bounding box size not saved server side

For some strange reason, the server will not save the max bounding box size no matter what and will always default to 1000 even though both the config file and in-game command saved the box size.

Server - Serverminer
Minecraft Ver. - 1.12.2
Modpack - SevTech: ages 3.1.7

Does it work if you stop the server completely before editing the configuration file?

Sadly, no. Everytime I edit the box size the server will go with the default 1000.