Minecraft – How to stop pillagers from spawning

Several things could cause this. There could be a slightly-too-dark corner in your tunnels or an overlooked side passage. If the light level drops to 7 or lower in any square, you need more torches.

In order to ensure >7 light in all areas, you need torches every 12 meters (blocks) in all directions. However, note that if your tunnel is more than 1 block wide, simply placing a torch on the wall every 12 blocks will result in a square that has less than 8 light at the midpoint on the opposite side of the hall. My recommendation is to be excessive in your lighting and place a lot of torches.

There isn't a "fix" to your worlds not saving. Just make sure press "Quit to title" before closing the game to make sure it saves the world. Every now and then, the game saves itself but you can never be sure when this happens.