Most things are completely broken in the spawn chunks. Hoppers stop transporting items, furnaces stop smelting them, pistons don't change when powered and entities don't get ticked. Basically any farm based on the spawn chunks won't work anymore.

What I expected to happen was...:
In a spawn chunk (and therefore loaded), everything should continue working as if a player is nearby or the chunks are forceloaded.

What actually happened was...:
Originally, I noticed this issue in my survival world where my furnace stopped smelting items while I was in the nether.

After some testing, I found out, that this is not exclusive to furnaces. Instead it actually applies to everything except redstone (only redstone itself e.g. dust, comparators, repeaters but also command blocks). Entities do not get ticked, pistons do not extend, hoppers don't transport items etc.
This is a serious issue to anyone using the spawn chunks to keep specific farms always active.

Steps to Reproduce:
For simplicity this just describes the original problem of furnaces not smelting. With some changes you can monitor most problems described (e.g. hoppers not transporting items)
1. Create a new world
2. Place a command block and a furnace above in the chunk you spawned
3. Put some items in the furnace so it begins to smelt
4. Put the command tellraw @a {"block":"~ ~1 ~","nbt":"CookTime"} in the command block and set it to repeat and always active (or create some other clock) - the chat starts to fill with increasing numbers from 0 to 200 and restarts after that
5. Build a nether portal and go through it (or run /execute in minecraft:the_nether run teleport @s ~ ~ ~ )
6. After a few seconds the value will stop increasing and stay the same until the world is reloaded or you go back to the overworld