In both drama and movie, the main characters are investigating a murder related to themselves.
How Urashima Eiji doubts himself reminds me of Kim Byung Soo who wonders about himself.
Both drama and movie try to lead the audience to doubt the main character as they doubts themself.
And then the plot twists presented in both also remind me of each other.

What is both drama main focus? Yess, friends to lovers

What they both have in common is that they tell the story of a female lead who is a hardworking woman that faces difficulties in her love life (but in LND the female lead quit her job at the beginning of the story), and a male lead who secretly likes her and is always there to accompany her.
Also, both female leads have more cheerful and passionate personalities while the male leads are more calm and stable.

The differences are, LND is more comedic ones and IDLYY is more melodramatic, although there is comedy tags in genre categories.

Both dramas are about brain experiment
Both male leads change personality after the experiment
Both male leads fall in love with the female researcher who observe them
Both the fl are forced to manipulate the ml for the sake of the experiment
Both male leads have painful past that involve their family

The differences are :
BME ml is suffering from mental illness but ANHW ml has an intellectual dissability

Both dramas are set during the Japanese occupation.
Both dramas tell the story of good friends who become enemies due to different backgrounds.
Both male leads work under the Japanese government as undercovers.
Both male leads have multiple identity.
In both dramas, first and second male lead have the same love interest.

In both dramas, the female lead enters the world of the novel that she is currently reading.
The main leads are enemies with each other at first, but they fall in love eventually.

The two story concepts have one thing in common, a reversed timeline.
The difference is that in My Tomorrow, Your Yesterday the female lead is the one who has the timeline reversed.
While in An Ancient Love Song it is the male lead who experiences it.

Youth? Check.
Journey of discovery? Check.
Friendship? Check.
Great chemistry between leads? Check.
Natural acting? Check.
Friends (that's basically enemies) to Lovers tropes? Check.
The story maybe not exactly the same, but you will find similar vibes in both