May fix itself if you bring up the main Steam client window, you may need to interact with it? Resize? Or re-render the store or library? Can you confirm?

Can not confirm anything at the moment, was not running Steam 24/7 for the last couple of weeks. The update screen appears when the Steam client is running and the update is out. In any other case, the update is handled automatically at client startup.

From my memories, the update window is not resizeable, no interaction with it would make the content appear.

Would be happy to test the Steam Library/Store interaction hypothesis the next time an opportunity comes up.

It would help a great deal if there was a way to trigger the "Self Updater" dialogue at an arbitrary time. Otherwise, any new updates may keep happening at times when my Steam client is not running, rendering the testing process extremely unreliable.

From what I've experienced, closing the initial update window (which is blacked out), and then selecting Check for Steam Client Updates.. from the Steam menu in the client window forces the update window to redisplay with the content visible.

It's a workaround. But consistently broken.

Thank you, we've identified the problem, this will be fixed in a future update (next Steam client beta cycle most likely).

Update popup appeared today, was black initially, until I switched focus to the main Steam client window with Friend Activity opened and then back. It worked, similarly to what @TTimo suggested.

To be clear, now that you've updated your client we think this won't happen anymore.

Looks good so far, the latest beta update showed up correctly!

Another update appeared with the changelog displaying just fine.

Since this https://steamcommunity.com/groups/SteamClientBeta/announcements/detail/3350135361891074312
Steam Beta Update Fixes client update dialog sometimes failing to display update notes,
@L1Q I believe it's safe to Close this Issue.

Third update in a row with no issue. Yeah, I think it's safe to call this fixed! Thank you!