This bug affects 1.17.1 most recently, but seems to go back as far as 1.13.

Screenshots attached show a world in 1.4.7, then upgraded to 1.17.1

Flower pots have had multiple storage formats through Minecraft versions. Original flower pots used the block data field to store the type, when slots ran out in later versions, this was changed to a block entity, and finally in 1.13 "flattened" to a unique block for each flower pot/plant type.

All flower pots created with block data, when directly upgraded to 1.13 and above (including 1.17), are given a default of potted_cactus. Flower pots created with a block entity are upgraded correctly. Note that if a world is first upgraded to 1.7-1.12, then to 1.13+, these versions give it a block entity which is correctly processed by 1.13+.

Edit: Added sample world created in 1.4.7.