Sorry, I didn't explain it well. Even with the old behaviour it was possible to check the tiles with the herblore skill which indicates if there was a plant or not. For example, if you have planted broad beans you couldn't see them yet, but you could check with the herblore skill to see that there actually were some. This doesn't work with sorrel.
So, the bug is that the herblore skill doesn't work with newly planted sorrel and, thus, the herblore skill works inconsistently since it behaves differently for different plants. I wasn't aware of that (why should it behave differently?) and this led to my confusion and to this thread.