Not sure if somehow I am late to the scene to mine the ore but many times they disappear when I land next to them.
I’m also having this problem. I have mining and herbalism on a druid and no mining nodes appear on the minimap. Herbal knots have no problems.
One of two things will be happening.
Most likely is that you’re just too late. Nodes were changed a long while back (at least as far back as Legion, possibly further) to persist after the first person uses them, letting multiple people use the same node instead of just one. It’s not a very long duration (maybe a minute or two) but it’s more than just disappearing immediately. However there’s zero indication that a given node has already been “tapped” nor how long it has left until it despawns, which can - and does - cause them to just disappear seemingly out of the blue if your timing is right (or wrong, depending on how you look at it).
Alternatively, and this has been rare in TWW in my experience, you could be in the wrong phase. It takes a moment for your character to correctly switch phases when moving about the world, and sometimes this is long enough that a node will linger long enough for you to very nearly interact with it. If moving a considerably distance away and re-apporaching causes the exact node to reappear, this is probably what’s happening.