How to reproduce:
1. Open the attached "1234652.zip" project
2. In the menu bar, navigate to Window > TextMesh Pro > Sprite Importer
3. Assign "AppleEmoji" as the Sprite Data Source and "sheet" as the Sprite Texture Atlas
4. Click "Create Sprite Asset", then "Save Sprite Asset"
5. Inspect the newly created asset and expand the "Sprite Character Table" and "Sprite Glyph Table" fields

Expected result: the tables are populated with cutouts of the sheet
Actual result: the tables are empty

Reproducible with: TMP 2.0.0 (Unity 2020.1.0a14), 2.0.1 (Unity 2019.3.9f1)
Not reproducible with: TMP 2.1.0-preview.1 (Unity 2020.1.0a14), 3.0.0-preview.1 (Unity 2020.2.0a7)
Could not test with: 2017.4 due to the missing Package Manager