Trend lag is the delay with which a trend weight follows a real change in your body weight. With the usual smoothing methods it is about one week.
It is not a bug but a direct consequence of smoothing. An average of past days describes the middle of that period more than its end. While you are losing weight, the trend therefore sits above today's reading, because heavier days are still in it. While you are gaining, it sits below.
Calm versus speed
Every method trades calm for responsiveness. Stronger smoothing gives a calmer line that reacts later. Weaker smoothing shows changes sooner, but also more spikes. You cannot have both at once.
Example: Your weight sat at 80.0 kg for four weeks, then the readings drop to 79.0 kg. After seven days, exponential smoothing, the default in Lean is law, shows 79.5 kg, while the 7-day average already shows 79.0 kg. With a steady loss of 0.1 kg per day, the real value after four weeks is 77.2 kg and exponential smoothing sits at 78.05 kg. These are worked examples.
What to do with it
Give every change at least two weeks before you judge it. A diet adjustment can only be assessed fairly after ten to fourteen days. For bulking and cutting, where you judge changes over weeks, the lag is not a problem. It only becomes one if you readjust after a few days or compare weeks calculated with different methods.