System Information
Operating system: Windows 10
Graphics card: Geforce GTX 1050 TI
Blender Version
Broken: ( 2.83.13, , master, 2020-04-25)
Worked: (2.82.7)
Short description of error
Grease Pencil Animation keyframes are not showing in 2.83
Exact steps for others to reproduce the error
Based on attached .blend file, Click on Animation Tab and choose Grease pencil on Bottom Menu. I have added 2 keyframes for grease pencil which is visible in 2.82 but not visible in 2.83.

