I also had this issue once after installing the latest beta on Windows 10.

A client restart fixed the issue for me as well.
So far it hasn't happened again.

I did clear my download cache, though.
Maybe that prevented the issue from happening again?

There's also this whole thing (certain Steam servers have / had issues with IPv6):
https://steamhost.cn/steamcommunity_com/groups/SteamClientBeta/discussions/0/612032374411202916/

Maybe it's related?

Yeah, I'm kinda at a loss. This happens *very* consistently (after about an hour or two of having the client open, but never anywhere close to "immediately"), clearing the download cache hasn't fixed this. Might be ipv6 related, as I do have ipv6 addressing, but I also don't think that that should be changing/affecting this in that timeframe.

i have a recording of this here: https://hecate.pink/assets/downloadbug.mp4

note that there is Nothing output to stdout when the downloads get wedged; it's just entirely unclear why this happens. i'm double checking now to make sure this is bisect-able between the stable client and beta client (will take another couple hours and some patches being available).

checking back in after a day: switching back to the non-beta client mitigated this (steam has been running for ~21 hours and still updates stuff fine, whereas before it would not work after 2 or 3 hours of idle), so this does appear to just be a beta client bug I am hitting. not download cache or ipv6 related necessarily.