Steps to reproduce:

1. Open attached project "779069.zip"
2. Open scene "repro"
3. In hierarchy, select "Canvas > Text"
4. In inspector, notice that Text field has two lines and Best Fit enabled
5. In scene view, notice that only the first line is displayed

Additional steps:
6. Disable Best Fit and notice that both lines are displayed
7. Re-enable Best Fit

8. Add new empty line to the text field
9. Notice that now two lines are displayed
10. Remove new line

11. Start adding symbols to the second line (but not new line)
12. In scene view, notice that symbols added to the second line affect the appearance of the first one by shrinking it. If enough symbols are added then the second line will become visible too.

Workaround: Add empty line at the end of text field

Not reproducible: 5.3.0f2
Regression introduced in: 5.3.0f3
Reproduced with: 5.3.3p3, 5.4.0b10