How to reproduce:
1. Add the attached project ("Provider.Submit Folder.zip") to your preferred workspace and open it
2. Connect it to Perforce and submit the initial project (Assets/Project Settings)
3. In the Project Window check out the "Test" folder (the folder's .meta file and the 3 materials within should get checked out)
4. Go to "Simple Window" -> "Submit" to submit the folder
5. Observe the Version Control window

Expected result: both the checked out asset files and their .meta files in the folder are submitted without issues
Actual result: only the asset files in the folder are submitted and the .meta files remain checked out

Reproduced in: 2020.1.0a3, 2019.3.0b2, 2019.3.0a4
Not reproducible in: 2019.3.0a3, 2019.2.3f1, 2018.4.8f1, 2017.4.32f1 (In these versions the folder contents aren't submitted at all)