An isolated black hole is subject to the evaporation of its mass due to the Hawking radiation caused by the vacuum fluctuations in its vicinity. This would seem to rule out the possibility of formation of bound states of primordial black holes. There are two conditions which need to be satisfied for a black hole to evaporate due to Hawking radiation:
(a) The black hole should be isolated.
(b) It sould be in field-free space.
Primordial (microscopic) black holes were produced in vast quantities in the immediate aftermath of the Big Bang. The conditions prevalent in that era were very different from those present today.
(1) Matter was highly compressed - all matter was closely packed together, and expanding at a high
rate. This violates the condition of isolation of black holes necessary for the Hawking radiation.
(2) Near the unification temperature, of the order of 1016 GeV, all the four fundamental interactions of nature are expected to have the same strength. In particular, the gravitational interaction would have a strength far greater than it has now. This violates another condition for the Hawking radiation, namely, a field-free space. This also means that the rate of gravitational interactions among the black holes would correspondingly be higher - and they would form stable, bound states.
In other words, extremely high number density, vastly stronger gravity and an enormously larger rate of interactions are likely to lead to the formation of stable bound states of primordial black holes.
This is analogous to what happened with neutrons - although a free neutron decays, neutrons in the
nucleosynthesis era of the early universe finding themselves in high number densities and subject to the strong interaction formed stable nuclei and never decay except those neutrons that are in heavy nuclei containing a large number of protons.
New research calls these stable gravitational bound states of PBHs "holeum", and leads to the
conclusion that they form an important constituent of dark matter.