Maybe you are in 3D mode and draw only 2D, then it it 100 % sure it will happen at a point, maybe you also have turned of the warning, else TC will warn you about it everytime it may happen.
Toggle to 2D mode down on Inspector bar if you should draw only 2D or open the selector Properties next to Toggle and set it to 2D/3D depending, then it should always draw on workplane if drawing 2D object's.
V18, V19, V20, V21, 2015/ 16/ 17 Pro. Platinum
Deluxe V20, V21, 2015/ 16/ 17
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB