Correct way to draw a carbene
Put simply, there's no precedent for this unusual depiction of a singlet carbene.

Your comment about most carbene depictions, for both singlet and triplet carbenes is true, but a bit lazy. Yes, many chemists, frequently just draw two electrons as a lone pair on the carbon for both. In principle, with a moment's thought, you can often deduce the likely spin state.

As described, by the comment above, the standard for 2D chemical structure depiction is the IUPAC guide Graphical representation standards for chemical structure diagrams



In short, there's a difference in representing a triplet (two unpaired electrons), which should show preferably two separate electron "dots."

For a singlet, the lone pair is indicated, emphasizing the connection between the electron spins.