Hello. I was instructed by Steam Support to post about this issue here.

I have background downloads disabled in Steam for Debian Linux.
I've noticed on several occasions that Apex Legends is getting updated during gameplay and this did not start with the latest major Steam client UI change.

Apex is set to use Proton Experimental.

I use the following launch options for Apex:
DXVK_HUD=compiler vblank_mode=0 gamemoderun %command% -cl_forcepreload +fps_max_menu 72 +fps_max 180

Thanks!

Context conversation with Steam Support:
https://imgur.com/a/2V7F04j

Me (OP):

Game launched via compatibility tool: Steam Play
I'm not sure if this is Linux only, but sometimes Apex Legends gets update massive patches while the game is running and Steam-wide settings for downloading in background is set to 'never'. The game becomes incredibly stuttery while the game is patching since it is run off mechanical hard drives. Is this a bug in Steam or intentional?

Jose:

Hello,

I understand your concern.

Currently, Apex Legends isn't supported with Steam Play so you're likely to encounter difficulties with the title playing it in this way.

Please submit all issues or bugs occurring when using this new feature, to the Valve - Proton Github bug database using a GitHub account. This database is regularly reviewed by the Valve development team.

In the meantime, the best source for further troubleshooting steps and suggestions can be found in the Steam for Linux community discussion as well as consulting our Proton - Quick-start Guide.

Steam Support,
Jose

Me:
No, it's a problem with the Steam client.

Jose:

I understand your concern however, as previously mentioned, Please submit all issues or bugs occurring when using this new feature, to the Valve - Proton Github bug database using a GitHub account. This database is regularly reviewed by the Valve development team.

In the meantime, the best source for further troubleshooting steps and suggestions can be found in the Steam for Linux community discussion as well as consulting our Proton - Quick-start Guide.

If you have anymore unrelated question, please let us know. 

Steam Support,
Jose

Thanks again! I am otherwise very satisfied with what Steam and Team are doing for gaming on Linux.