The books of Merton’s that were published in his lifetime are generally very good. After his death, a lot of writings were gathered together & published. Many are nowhere near the quality of his earlier work.
I confess to not knowing what may have happened–Whether he, as you say, went off the deep end, or if it is just that his publisher took advantage of his popularity, & published some random thoughts of his, that he would not have approved if he were still with us.
He apparently became very interested in Eastern religions back in the late 50s & into the 60s, when he passed away. So i don’t know if this was just a passing fad. (In the 60s, it seemed like half the people I saw were “interested in Eastern religions”, & the other half were interested in rock& roll, and the drug culture.)
I have always thought it was very sad that the impression that was left of him was nothing like the wonderful insights of his early writings.