Why does the app change "Continue selling when out of stock" on my products? (Essent Preorder Back in Stock app)

That's expected. A preorder is technically a sale at 0 stock, so Shopify's Continue selling when out of stock must be on for it to work. With the default Automatic (recommended) mode, the app manages this setting for every variant in a Live campaign:

• When product is out of stock and Always: turned on.
• When product is in stock: turned off.
• When you unpublish a campaign, the app asks whether to turn it off again. Choose Yes in most cases.
• When a stop-selling limit is reached, the app turns it off for that variant.

The app also turns on Track quantity for campaign variants.

Want to control it yourself?

Open the campaign → Preorder products tab → set Continue selling to Manual. The app then never changes it, except to apply a stop-selling limit. If you don't want a product on preorder at all, remove it from the campaign.

Every change the app makes, and why, is listed under Home → Activity log.

Full guides: Continue selling when out of stock and Essential Preorder and Activity log.