I'm going to wager that it has too many points. You'll note that other similar bodies of water (e.g. North Saskatchewan in Edmonton) are made up of several Places, rather than one continuous one.

It used to be that large Places disappeared (e.g. Banff/Jasper national parks, Cold Lake Air Weapons Range), but that seemed to be fixed.

Only other thing I can think of is that someone else deleted it... was it locked?

It was locked R4. I don't think it was deleted. There have been tile updates since. I had done it as a couple different places, but I was also getting save errors every 25 edits (not geo-nodes)... I've noticed a lot of those errors again recently. Happened this time last year too, if I remember correctly.