Steps to reproduce:
1. Open attached project "text_test.zip"
2. Open "Repro Negative UV Values" scene
3. Click "File/Save Scene"
4. Notice how material references get lost and ALL text(with the same material) becomes invisible ("bug1.gif" attached)
-If you change text alignment it becomes visible again
5. Change "TextMeshPro" objects material to "ARIAL SDF - Outline"
6. Save Scene
7. Notice how text with changed material doesn't disappear ("bug2.gif" attached)

Also, I recommend watching https://www.youtube.com/watch?v=ZkYvxUJBNMg It perfectly illustrates this bug

Reproduced with: 5.4.0f1, 5.4.0f3

Not reproducible with: 5.2.4f1, 5.3.6f1