I am also looking for it, it is indeed sad. It is not in "Da Ming Palace Ci Original Sound Collection". The songs "Imprint" and "Loneliness" are very similar, but not the same.

It is recommended to listen to the "Princess Taiping Original Soundtrack". Although the songs in it are "Long Stay, Hot Spring Water, Death" and "The Original Soundtrack of Daming Palace Ci" corresponding to "A Tea Stove Warmth, Illusion, and Prosperity" "Behind" has the same melody, but the sounds are different. The tunes of "Daming Palace Ci Original Soundtrack" sound sharper, while the "Princess Taiping Original Soundtrack" is smoother.