When you load into a new realm (your Abeyance for example), the world will start out with the 350 realm item limit being somewhere in the 6-10 range, definitely non-zero in most cases. As you begin moving around in the realm and different far away areas start loading in, that number begins going up as poi's and chests begin loading in. These include loot chests and seemingly other POI placed objects.
In the video below, you can see the limit at 20/350 and as I approach the W.S. monument, the limit starts creeping up, and when I place a bed that number jumps to 67/350.
When I delete the next bed at W.S.'s place, the number goes from 69 back down to 29. So it seems that just deleting or removing any built item will get the number to reset to the old "limit" with the POI/chest items included (and the number jumping for new/placed items is a separate issue).
Then you can see as I run further around in new unexplored areas, the number climbs further to 44, which will be the new number for the realm (until I explore even more of it, and it goes up further).
If you are close to your 350 limit, or are really low, and then go exploring more of your abeyance if you haven't gone to all of it, that /350 number can go up even further on it's own.
*Edit:
I hadn't tested re-loading the game yet, and when I did the limit went back down to 12/350, and immediately started climbing again as I began moving around. It appears to be a per-session issue where it loads everything back in.
So if you already have a high item count, you could hit the 350 limit fast by moving around, and you will need to reload the game to get it to go back to the old number. (or maybe travel out to a new realm and wait > 10 minutes for the server to reset your abeyance, then go back)
*Edit 2: further info from @Severyn below: #1215018539912658965 message
Link to seekplan bug: https://playnightingale.sleekplan.app/feedback/138556