The reason we dont say Tachanun by Ma’ariv – is for the “meorer dinim” reason – no source in Halacha.
Originally, Tachanun was the MAIN part of Davening – as explicit in Daniel – literally falling on the floor 3 times a day. But, because of the “meorer dinim” reason, we dont fall on the floor (the sphardim dont fall at all – the ashkenazim fall on the table – but not on the floor anymore – which is the real Halacha)
All this is because of “meorer dinim” reason.
Most Chassidim in LKWD are VERY makpid to say Tachanun, with 13 Midos, every day Shachris and Mincha – even after Shkiya.
But, i guess, fpr some since the minhag ashkenaz is to fall on the table, they extend the “meorer dinim” reason even more.
BUT EVERYONE TODAY HOLDS OF THE “meorer dinim” reason – IN ONE WAY OR ANOTHER – AGAINST HALACHA!