I was trying to create an advancement which requires you to place every possible type of plant inside a flower pot. One of the criteria looks like the following:

"potted_dandelion": {
    "trigger": "minecraft:placed_block",
    "conditions": {
        "block": "minecraft:potted_dandelion"
    }
}

However, it didn't trigger, and this led me to discover that even using just a simple placed_block trigger without the condition as follows:

"potted_dandelion": {
    "trigger": "minecraft:placed_block",
},

This will not trigger upon placing anything inside a flower pot, but it does trigger upon placing other blocks as intended.