Why is my Slack DM list incomplete?

I am trying to keep the full list of people in the "Direct Messages" sidebar, in order to permanently be able to see who's online and start a conversation with these people when I need to.

I could accomplish this by clicking "Open a Direct Message" to add each member, but Slack actually keeps reducing this list in a mysterious way, despite the setting "in the sidebar, show all my channels and DMs".

Does anyone have an idea how to fix this?

Here's what I got from Slack support:

I can confirm that what you’re experiencing is expected behavior, which I’m happy explain. Once you have more than 11 direct messages (DMs) or group DMs open, any additional direct messages are expected to fall off the list if the DM is not opened during your current session, there has been no activity in the DM in the past week, or there are no unread messages in the DM.

I’m afraid there’s currently no way to turn this behavior off, I’m sorry for the frustration this has caused you. I certainly can see how it would be useful to be able to view all your DMs listed in your sidebar. I’ve gone ahead and shared your experience with the right people on my team.

In the meantime, you can star all your key DMs to keep them all listed in your sidebar (:star: icon under a member’s name at the top) or quickly pull-up a conversation by using the quick switcher, Cmd/Ctrl + K.

• Star channels, messages, or files: https://get.slack.help/hc/en-us/articles/201331016-Star-channels-messages-or-files

• Using the Quick Switcher: https://get.slack.help/hc/en-us/articles/226599368-How-to-use-the-Quick-Switcher-