Separation anxiety is a milestone

Between six and nine months, most babies work out that objects and people continue to exist when out of sight. The same insight that makes peekaboo hilarious makes your exit distressing.

Short, honest goodbyes work better than slipping out. Say the same phrase every time, leave, and come back. The pattern of leaving and returning is the thing being learned.

Practising with play helps. Hide a toy under a cloth, hide behind a door, let your baby find you. Controlled versions of separation make the uncontrolled ones easier.