Adding constraint on InnoDB-Table locks whole database, although an index for that constraint already exists

We are running a Galera-Cluster with 3 Nodes.

Someone accidently deleted all foreign-keys of a table. The corresponding Index-Entries remained.

Because the Index-Entry did exist we assumed that we can alter the table to add one of the missing foreign-keys using "total order isolation" without a global lock.

But the database was locked for hours. It would be nice, if creation of foreign-keys would not block the whole database when an corresponding index already exists.