Creative mode obtained Bucket of Axolotl/Tropical Fish only spawns one kind axolotl/tropical fish

You can work around this bug using spawn eggs, which give a random variant each time they're used.

You get a bucket with a fixed variant by adding an NBT tag with the /give command:

/give @s minecraft:tropical_fish_bucket{BucketVariantTag:235340288}
/give @s minecraft:axolotl_bucket{Variant:4}

You can visit the wiki pages https://minecraft.fandom.com/wiki/Tropical_Fish#Entity_data and https://minecraft.fandom.com/wiki/Axolotl#Entity_data to get the numbers for the variant you want.

The problem with spawn eggs is the tropical fish despawn after travelling a certain distance; using a bucket ensures that they don't despawn.

you can specify the FromBucket tag in a spawn egg using the /give command:

/give @s minecraft:tropical_fish_spawn_egg{EntityTag:{FromBucket:true}}
/give @s minecraft:axolotl_spawn_egg{EntityTag:{FromBucket:true}}

Can confirm in 21w16a.

Can confirm in 21w17a.

Can confirm in 1.17.

confirm in 1.17 full release.

can confirm for realms.

The bedrock one was fixed but the Java still not fixed!