Madhur Jaffrey uses lemon juice in her Murgh Makhani recipe (first book, linked to her 1980s BBC TV series). It was the first Indian recipe I ever made (years ago) which had me think "wow!".
I see CTM, Makhani and butter chicken as all closely related, so I'm not surprised to see lemon juice. I have, however, added lemon juice to some dishes and it variously improves/ alters/ruins them so perhaps it's worth adding lemon juice towards the end for a clear before and after assessment. As CA says, they'll likely already be some lemon juice coming in from the tikka marinade.
I did chemistry at school and this included some 'quantitative analsis', like working out how many carbon atoms were in your test tube, or something like that. I forget, but it would be handy to know about quantitative analysis as applied to food elements. Like some labs could tell you if a food sample contained any trace of cyanide, salmonella, Sudan1 dye or whatever. I wish I knew how to take a sample of CTM from my local BIR and analysis it to confirm conclusively if it included and how much - lemon, garlic, MSG and and other key ingredients which I wanted to check for.
Regards
George