One cannot reference another sheet in a formula by using the UI.

To replicate,

1. Open your spreadsheet demonstration app.

2. Create two work pages.

3. Enter some information on the first page.

4 .go to the second page, select any cell, and press =

5. navigate to the first page and select a cell with data in it.

6. Check the second page again, there is no reference link.

Workaround: manually type the sheet name in the formula