Normally, an amethyst geode needs a lot of blocks to spawn in bc it's an underground structure. But it can generate in only a block if spawn by "/place".
STR:
Step 1: Load in a world, superflat is recommended.
Step 2: Place the first "Always Active" repeat command block that said: "/place feature minecraft:amethyst_geode ~ ~10 ~".
Step 3: Place the second "Always Active" repeat command block that said:
"/setblock <X-pos-of-1st-repeat-command-block + 5> <Y-pos-of-1st-repeat-command-block + 15> <Z-pos-of-1st-repeat-command-block + 5> <block>" (For example if the 1st repeat command block (the repeat command block that said "/place feature minecraft:amethyst_geode ~ ~10 ~") is in 0 -60 0 then since 5 is 5 more than 0 and -45 is 15 more that -60, the 2nd repeat command block will said "/setblock 5 -45 5 brain_coral_block")
, then wait. -> You will see that an amethyst geode will spawn
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I already give you the steps to reproduce, despite it kinda confusing.

I don't see a bug here. Geode generation has a lot of issues but I wouldn't consider it a bug if a geode generated seemingly floating inside a large cavern, just a rare occurence. (And in any case this report doesn't even include an example of that, it only states that it's theoretically possible.) And as for /place, it's a wonky command which afaik is only intended to be used for debugging particular structures/features; it includes no more functionality than is necessary to accomplish that. To change this behavior with the geode would be to unnecessarily change the code for generating a geode, with the potential to create new worldgen issues when geodes already have so many.

Unfortunately, this is not an ez question to answer, while obviously the "observed results" is "You will see that an amethyst geode will spawn", the "expected results" is much more difficult and not just simply the opposite of "observed results". Because if it is, the issue of amethyst geode generates in two blocks will still occur, and so does three, four, five, six and so on. Which means we have to answer the question: "What is the minimum amount of blocks for an amethyst geode to spawn in. Unfortunately you cannot just use Calculus to solve this because the size of amethyst geode generate at random and there's no function to consider. The only way to completely fix this is to pre-determine the size of the amethyst geode before generating, then see if the place it wants to generate is "fit" or not. But the problem is it might cause more issues and it sounds like a feature request. So in conclusion, the "expected results" is "I'm not sure".
Fair, marking this as plausible for Mojang to decide. However, without an "expected result", I'm not sure what the aim here is, and as @unknown already pointed out, I don't think this is a bug (or worth fixing).

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