Steps to reproduce

send a video with send_media_group including one input_media_video with a local video and local thumb image passed as open('path','rb').
thumb image width is less than 320px and size is about 40kb.

Expected behaviour

video sent with image as thumb.

Actual behaviour

video sent with no thumb (white background).

note 1: the video sent properly with send_video function and the same thumb.
note 2: video size should be more than 10-11mb so telegram use the passed thumb image, instead of it's own generated thumb.
note 3: you can use edit_message_media instead of send_media_group. this problem is related to input_media_video, so any function which has this, is effected.

Configuration

Operating System:
Ubuntu 18.04

Version of Python, python-telegram-bot & dependencies:

python-telegram-bot 12.3.0
certifi 2019.11.28
future 0.18.2
Python 3.6.8 (default, Jan 14 2019, 11:02:34)  [GCC 8.0.1 20180414 (experimental) [trunk revision 259383]]

Logs

no logs.