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Describe the Bug:
After casting Conversion mountains turn into plains but during opponent's turn they turn into mountains again


Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
October 2022 4040bdb / sealed

What exactly should be the correct behavior/interaction?
Mountains should stay as plains while Conversion is in play.

In fact the lands change all the time from one type to the other! It seems that every new action (cast, activate, etc) changes the type back and forth. I have no idea what has caused this.

Fixed this, but making the whole system work better to add and remove basic land types (and the very important interaction between Blood Moon and Conversion)

Fixed in b7eafbc

This seems to work ok in simple tests for ( I didn't test any interactions ) for cards like Conversion that have a set type to change to another set type. But for Illusionary Terrain and Vision Charm things don't work correctly.
1) Illusionary Terrain changes all lands to the land type chosen secondly.
2) Also it delays to change the type for an action or step so after it enters the battlefield, if you tap a land it will still produce mana of what the land is originally, Name changes instantly. This doesn't happen for Conversion.

1) Illusionary Terrain changes all lands to the land type chosen secondly.

Illusionary Terrain
{U}
Enchantment

Cumulative upkeep

As Illusionary Terrain enters the battlefield, choose two basic land types.

Basic lands of the first chosen type are the second chosen type.

Vision Charm

Instant
[...]• Choose a land type and a basic land type. Each land of the first chosen type becomes the second chosen type until end of turn.
[...]

So it's only basic lands of the first type that change into the second type for Illusionary Terrain, and only lands of the first type for Vision Charm. I must be misunderstanding something.

2) Also it delays to change the type for an action or step so after it enters the battlefield, if you tap a land it will still produce mana of what the land is originally, Name changes instantly. This doesn't happen for Conversion.

I have subtypes recalculating before iid is recalculated, which is backwards. Fixed to recalc after "recalc" in get_abilities(... EVENT_CHANGE_TYPE)

Illusionary Terrain with your fix changes all types of basic lands to the chosen type. Not only the basic type of lands chosen at first choice.

Should be all shored up in 54c4e4d

I just realized that Blood Moon was bugged the same way, non basic lands keep switching back and forth from mountains to their original nature.
Want me to create a separate post?

No, this is presumed fixed in dev. Changing the title now. Thank you