How to reproduce it:
1. Open the attached project "Piratales.zip"
2. In the Project view select an "Attack.psb" file and in the Inspector window select the Sprite Editor
3. Select the Skinning Editor module from the dropdown menu in the Sprite Editor window
4. Select any Sprite by double-clicking on it
5. On the right Sprite Editor's window side, select a Weight Slider button
6. Select any vertex and change its weight using the Weight Slider
7. Click on the "Apply" button in the Sprite Editor's windows toolbar
8. On the right Sprite Editor's window side, select a Create Vertex button
9. Create a Vertex on the Sprite
10. Click on the "Apply" button in the Sprite Editor's windows toolbar
11. Observe the added Vertex on the Sprite

Expected result: Vertex changes are applied
Actual result: Sprite Vertex changes are not applied. Created Vertex is removed

Reproducible with: 2020.2.0a14, 2020.3.2f1, 2021.1.1f1
Not reproducible with: 2018.4.33f1, 2019.4.23f1, 2020.2.0a13
Could not test with: 2021.2.0a11 - Sprite Editor doesn't open

Notes:
- Only reproducible when Sprite is imported from the .psb file using 2D PSD Importer
- Regression version doesn't depend on 2D Animation and 2D PSD Importer versions. E.g. this issue is not reproducible with 2020.2.0a13 with 2D Animation 4.2.8, 2D PSD Importer 3.1.7, and reproducible with 2020.2.0a14 with the same packages and their versions
- In 2021.2.0a11 appears error: "NullReferenceException: Object reference not set to an instance of an object. UnityEditor.U2D.Sprites.SpriteFrameModuleBase.OnModuleActivate() "