Whenever all of the players in our realm move far enough that a furnace is no longer in sight the furnace stopps smelting whatever item is in it. We also have an underground rail system that is over 1,000 blocks long with consistent power rails and redstone blocks, however, when we send off a chest mine cart, the mine cart will travel out of vision and then when we stay in the area for 10 minutes mining and walk in the direction we sent the mine cart, it immediately come back into view and starts moving again until it’s out of sight, then stops again and repeats the process. We have also noticed that when we aren’t in the vicinity of our farm the crops do grow much if at all. We think all of these may be connected by a bug or we are also playing on a realm with 5 add-ons and think this may be affecting the way the game functions but either way I know it needs to be bought to your attention and fixed. The add-ons we have are:
Mine Buddy Add-On
by Blockception
Prizma Visuals
by Starfish Studios
Farming Add-On
by Podcrash
Realism Biomes Add-On
by Block Factory
Furniture Add-On 2.0
by XP Games
A video can be supplied if needed
Environment
Seed is -1359905628689007903
You are describing normal game behavior. Minecraft worlds are divided into chunks measuring 16x16 blocks. Only loaded chunks are active at any given time. This is based on distance from a player. If an entity enters an unloaded chunk, it is also unloaded and frozen in that state until the chunk it entered is loaded. Simulation distance is set by either the server you are on or your world settings if it is a local world. This is not the same as render distance which is how far away blocks are loaded into view.
I would recommend looking at ticking areas if things like crop growth and redstone operation need to continue working. Some things like mob spawning do not work in ticking areas as that only occurs in player-based simulation areas.
Ticking areas may not be desirable in all situations, but you should be able to read enough about them to determine if you would like to try them.