I use the /forceload command to load certain chunks on my server. When I restart the server (e.g. by re-entering a single-player world) these chunks are not loaded automatically.

I have to fly toward the chunks until get loaded and then they stay loaded.

Recreated by:

• Open a new single-player world.
• Add a repeating command block saying "hi" somewhere.
• There will be "[@] Hi" in chat.
• /forceload that chunk. (/forceload add ~ ~)
• Fly far away s.t. that chunk would normally unload.
• There is still "[@] Hi" in chat.
• Exit the world.
• Re-enter the world.
• There is no "[@] Hi" in chat.
• Fly towards the command block and it will again say "[@] Hi" in chat at some point.
• You can then fly away again and there is still "[@] Hi" in chat.