It's due to Hawking radiation, a black body radiation due to quantum effects near the event horizon of black holes and named after Stephen Hawking that predicted this phenomena. From the same Wikipedia article:
Hawking radiation reduces the mass and the energy of the black hole
and is therefore also known as black hole evaporation. Because of
this, black holes that lose more mass than they gain through other
means are expected to shrink and ultimately vanish. Micro black holes
(MBHs) are predicted to be larger net emitters of radiation than
larger black holes and should shrink and dissipate faster.
I don't think it was ever observed to actually lower mass of black holes that we observe as astronomy phenomena, the process would simply take too long for any measurable effects to manifest, but scientists did try to observe it experimentally. In September 2010, Belgiorno et al. claimed they've:
... produced Hawking radiation by firing an intense laser pulse through
a so-called nonlinear material, that is one in which the light itself
changes the refractive index of the medium.
But this is yet to be reproduced and confirmed by experiments of others.