Probably this issue is related to unsupported video encodings: #1464
$ ffprobe UYW_Title_Splash.mp4
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/home/tumppi/.steam/debian-installation/steamapps/common/Use Your Words/uyw_Data/StreamingAssets/Videos/Splash/UYW_Title_Splash.mp4':
Metadata:
major_brand : mp42
minor_version : 0
compatible_brands: mp42mp41
creation_time : 2019-07-11T21:41:41.000000Z
Duration: 00:00:04.47, start: 0.000000, bitrate: 1821 kb/s
Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 1280x720, 1479 kb/s, 30 fps, 30 tbr, 30k tbn, 60 tbc (default)
Metadata:
creation_time : 2019-07-11T21:41:41.000000Z
handler_name : ?Mainconcept Video Media Handler
encoder : AVC Coding
Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 317 kb/s (default)
Metadata:
creation_time : 2019-07-11T21:41:41.000000Z
handler_name : #Mainconcept MP4 Sound Media Handler
Here is the log file with Proton Experimental (1629749691 experimental-6.3-20210823) and GST_DEBUG=4: steam-521350-gst_debug.zip
The intro videos work with Proton 7.0-1! But the gameplay (subtitle) videos are still rendered with the placeholder.
The log file with GST_DEBUG=4: steam-521350.log.zip
It's getting better! Tried with Proton 7.0-6 and some of the subtitle videos work perfectly but some videos look like this (audio works ok):

I ran ffprobe to working and non-working video files and the non-working videos share this data:
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'clip.mp4':
Metadata:
major_brand : mp42
minor_version : 512
compatible_brands: isomiso2avc1mp41
creation_time : 2017-02-07T03:51:00.000000Z
encoder : HandBrake 0.10.5 2016021100
Duration: 00:00:07.00, start: 0.000000, bitrate: 1173 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 910x512 [SAR 4096:4095 DAR 16:9], 1005 kb/s, 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc (default)
Metadata:
creation_time : 2017-02-07T03:51:00.000000Z
handler_name : VideoHandler
vendor_id : [0][0][0][0]
Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 163 kb/s (default)
Metadata:
creation_time : 2017-02-07T03:51:00.000000Z
handler_name : Stereo
vendor_id : [0][0][0][0]
The log file with GST_DEBUG=4 where the issue occurred: steam-521350.log.zip
Same bug with Steam Deck and default proton settings at this date.
No problem with the game excepts the videos that look like the previous post.
proton 7.0-6x1 2023-02proton 7.0-1x1 2022-02proton experimentalx1 2021-08
Compatibility Report
System Information
I confirm:
Log file: steam-521350.zip
Log file with
GST_DEBUG=4: steam-521350-gst_debug.zipSymptoms
Reproduction