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Reproduction steps:

1. Have nothing on a platform.
2. Expand the plaform like 40 tiles south.
3. Order deconstruction of everything on the platform.
4. Observe: There are artefacts left on the map.

Reading 118940 this technically might not be a bug, but it is annoying. If you expand your platform and then shrink it, these artefacts will be left there for eternity.

Could you just scan map when platform shrinks? And maybe, in migration to new version, rescan all platforms so it applies to existing saves automatically?

This is so annoying and is definitely a bug (but the dev's disagree, see 118405)
When this happened to me, I just ran
Or, you could try putting this in your saves control.lua: Then everytime you load your save, it will refresh the map.
You could even try putting stuff in a lua event handler to make it happen more frequently.

Ok I've made a mod: https://mods.factorio.com/mod/platform-map-update
You can also put the code in your control.lua.

It's not that good as you have to open and close remote view (or switch planets) to get it to update.
I could also make it update every few ticks or so, perhaps that is better?
Or maybe I can find a lua event that triggers when ghosts are created or destroyed, and use that.

Wait, why is this in Technical help? Did I misclick? Could somebody move it to Bug reports?

IsaacOscar wrote: Tue Dec 10, 2024 1:28 pm This is so annoying and is definitely a bug (but the dev's disagree, see 118405)

Our cases are not the same. Linked case 118405 is about ghosts, this is about built entities.

And the final comment:

Rseding91 wrote: Tue Nov 19, 2024 2:25 pm All surfaces have "fog of war" because to not have it means constant re-scanning of chunks to update the map images which has a measurable performance overhead.

Doesn't apply either. This doesn't need constant rescanning. This only needs to rescan a chunk on a space platform when a tile is deconstructed.

Brambor wrote: Tue Dec 10, 2024 2:39 pm I don't think our cases are similar. Linked case 118405 is about ghosts, this is about built entities.

Yes your right, very strange.
My guess is the tiles where deleted before the game did a rechart of the area.
But then when the game got to recharting, it only does it for areas near existing platform tiles, and so it didn't update that area.

I would still try my workarounds to see if it fixes it (in which case it is a charting issue, and not an invisible-platform tile issue)

Brambor wrote: Tue Dec 10, 2024 2:39 pm And the final comment by Rseding91:

Rseding91 wrote: Tue Nov 19, 2024 2:25 pm All surfaces have "fog of war" because to not have it means constant re-scanning of chunks to update the map images which has a measurable performance overhead.

Doesn't apply either. This doesn't need constant rescanning. This only needs to rescan a chunk on a space platform when a tile is deconstructed.

Oh I'm sure theres an efficient way to do this (rechart everytime a tile/ghost gets deleted, but skip some recharts if too many are getting deleted in a short period of time).
The dev's just have more important things right now, hence the report I linked being in minor issues.

In sandbox, you can "be" there with the invisible avatar. That rescans it and the tiles disappear.

I am sure it is related to scanning/radar coverage. Just like this 120644. But this one is easily reproducible and fixable and wrong.

Developers state there:

Rseding91 wrote: Wed Nov 27, 2024 9:04 pm So, you probably don't know how the player radar range works (I don't know why anyone except someone looking at the code would). Every 32 ticks a 2 chunk radius around the player is updated (chunk you're on + 2 in all directions). All changes done to the chunks for the next 32 ticks will not be reflected on the map if the player moves away from it before the 32 tick re-scan time is up.

I guess every tile on space platform makes the chunk it is in radar covered (kinda, but written more efficiently). This coverage is updated every 32 ticks. When you deconstruct a line like I did, the tiles are deconstructed, there are no more tiles in the chunk, so the chunk is no longer updated. This happened within the 32 ticks map didn't update, so the tiles are left on the map.