When you unmerge a platform account from your Behaviour account, your unlocked items, perks, characters, character levels, and player level will be returned to the platform account in the same state as when the account was originally linked.
Any remaining Auric Cells purchased on that platform since merging, or items purchased using Auric Cells, will also be returned to the platform account. Bloodpoints, Iridescent Shards, and Auric Cells unlocked through gameplay will remain on the Behaviour account. Any unlocked characters that were transferred to the unmerged platform account will be unavailable to the Behaviour account (unless repurchased).
If unmerging the account would leave the Behaviour account with NO remaining platform accounts, then all Bloodpoints, Iridescent Shards, and Auric Cells remaining on the Behaviour account will be granted to the final platform account that was unmerged.
Re-merging your account will not restore this lost progress.