It seems that a deleted or corrupt map I had bookmarked was causing this. Head to your bookmarks on Waypoint under Community > Content Browser > My Bookmarks. Or just go here: https://www.halowaypoint.com/halo-infinite/ugc/bookmarks
Find any map slots on the bookmark page that have no name. Click the bookmark icon in the bottom right so it goes back to an outline then restart Infinite. That ended up working for me.
I appreciate the attempt to help. Did not work for me. My forge map disappeared and my bookmarks are not loading. This is really crappy.
no dice, I go there and 13 of 56 bookmarks show up. is there a larger thread regarding this issue? feels odd that this would only effect a small player base but i’m not seeing any other post.
I made a post about this a week ago or so as well. I cannot even see all my bookmarks on the Halo Waypoint website so unbookmarking and rebookmarking isn’t even an option for me. It is very frustrating!
I kinda gave up on bookmarks till they fix it. Tired of selecting bookmarks and not seeing them. 
I ended up ctrl-clicking on the bookmarks that were showing so they open in a new tab, then on the main page I unbookmarked all that were showing. Then I refresh and I get a new page of 13 bookmarks and do the same until I find one with no name. After I found one with no name I unbookmarked it, checked on my xbox and I could see all of my remaining bookmarks so then I went to each of the tabs I had opened and rebookmarked them.
This is still an issue… A major one for me because all I really play Halo Infinite for nowadays is customs and not being able to see bookmarked maps has really but me off from playing this game. They need to fix this ASAP.
I host Halo LAN parties every month out of my house. I’ve been doing this for 20 years. During our last LAN in February, all 6 pages of my bookmarked maps were missing. I’ve tried the method suggested above to fix the issue, but only 13 of my 87 bookmarked maps are showing. 343 needs to fix this issue as soon as possible.