This information we have from Śrīmad-Bhāgavatam (10.2.32),
wherein it is said, patanty adhaḥ, which means “they again fall down.”
The monist philosopher, after executing severe austerity,
merges into the impersonal effulgence of the Lord,
but the living entity always wants reciprocation in loving affairs.
Therefore, although the monist philosopher is elevated
to the status of being one with the effulgence of the Lord,
because there is no facility for associating with the Lord and rendering service unto Him,
he again falls into this material world, and his service propensity is satisfied
by materialistic welfare activities like humanitarianism, altruism and philanthropy.
There are many instances of such falldowns,
even for great sannyāsīs/renunciants in the Māyāvāda/all is illusion school.
As soon as one thinks, there would be something
outside of Krishna´s control,
one lives in uncertainty and is ruled by fear.
Since Krishna rules everything and is present everywhere
and nothing happens without His consent,
FEAR IS OBSOLETE, because one is sheltered in the softest hands.
The one who is afraid, has long forgotten
and not yet dared to consider this view ...