Privileges discrepancy
You're logged in as rds_user, but you may not actually be logged in as 'rds_user'@'%'.

Privileges are actually granted not just per-username, but per username/host combination.  When you log in, you may be logging in as rds_user associated with a specific host, and that user may have less privileges.

Log in and run SELECT CURRENT_USER(); to see which user you're actually logged in as.

Run SELECT * FROM mysql.user WHERE user = 'rds_user'; to see all the users you could be logged in as.