core: Cast throws "resolvable URL" while it is

The problem

While using cast integration with Google Cloud, sometimes I encounter and error while integrations works. My URL have proper SSL cert (both inside and outside) and all five tries of casting TTS to Google Mini worked well. Error below.

Both errors count, but the one I wanted to report more is: Failed to cast media https://www.home-assistant.io/images/cast/splash.png. Please make sure the URL is: Reachable from the cast device and either a publicly resolvable hostname or an IP address - it appeares randomly in log, while I don’t even try to cast splash, neither I can - because it’s speaker device.

Environment

• Home Assistant Core release with the issue: 2021.1.4
• Last working Home Assistant Core release (if known): n/a (happened since few releases back)
• Operating environment (OS/Container/Supervised/Core): Supervised 10 Debian
• Integration causing this issue: cast
• Link to integration documentation on our website: https://www.home-assistant.io/integrations/cast/

Problem-relevant configuration.yaml

n/a

Traceback/Error logs

Logger: homeassistant.components.cast.media_player
Source: components/cast/media_player.py:348
Integration: Google Cast (documentation, issues)
First occurred: 9:25:02 (2 occurrences)
Last logged: 9:25:30

Failed to cast media https://www.home-assistant.io/images/cast/splash.png. Please make sure the URL is: Reachable from the cast device and either a publicly resolvable hostname or an IP address
Failed to cast media https://[redacted].org/api/tts_proxy/0[redacted]0_en-us_8[redacted]e_google_cloud.mp3 from internal_url (https://[redacted].org). Please make sure the URL is: Reachable from the cast device and either a publicly resolvable hostname or an IP address

Additional information