I am uncertain if this is exclusive to those two biomes. There are areas where you cannot place anything in these biomes, as if blocked by some already placed invisible item - you cannot interact with anything at these spots. At first I noticed this when I had the geometric placement mod activated, so I tested it without any mods and it still exists.
At first I thought these spots may be where sparkling pools might spawn, but I don't see why they would be in the tall grass biome. Also if you fully use up a sparkling pool then you can place a fire pit where it was.
Start a new game, no mods, default everything. Craft any placeable item and you can use it's placement sketch to find these spots in both biomes (possibly others too) where there is seemingly nothing but you are not allowed to place anything. [In the example pictures I did spawn in a walking cane and coffee to facilitate moving around the map.]