1.21.7 rc1 has added new sound events, textures, an item, and a jukebox song, but both the resource pack format and data pack formats were not incremented.
This means that a pack made in 1.21.7 that uses these new features would not work in 1.21.6, despite both versions sharing a pack format. Without being able to use pack overlays to get around this, it means there are no workarounds to avoid incompatibility.
Examples:
Item tag that includes the new music disc item
Item component that references the new sound event directly (without using an inline sound event definition, which is different)
Functions that reference the new item or new sound event directly
Item component that references a new texture (such as the camera overlay of the equippable component)
It should be noted that when moving from 1.21.6 to 1.21.7 rc1 this is not a breaking change, but the pack format is meant to work both ways, and it is indeed a breaking change when trying a 1.21.7rc1 pack in 1.21.6, despite both updates sharing pack formats.