Overworld spawn chunks are not loaded in the Nether; this has bought a lot of inconvenience as command blocks are not loaded in the spawn chunks in the overworld and hence cannot fire.
MC-99673
Overworld spawn chunks not loaded in the Nether
Resolved
Works As Intended
0
This is a feature suggestion rather than a bug report.
However, to clarify: the spawn chunks are not unloaded. Entities and tile entities are simply no longer sent update ticks after 15 seconds of no players being in the overworld. Not all tile entities rely on update ticks though, such as command blocks. Command mechanisms that do not rely on certain tile entities nor regular entities will continue to function.
If you want to ensure the spawn chunks are always fully functional, you can send an entity to or from the overworld via a nether or end portal, which resets the 15-second timer. For example, you could have a clock in the spawn chunks that runs only when no players are in the overworld, which drops an item entity into an end portal every 14 seconds. This will keep the spawn chunks fully functional.