  Reactor neutrino experiments play a crucial role in advancing our knowledge
of neutrinos. A precise measurement of reactor electron antineutrino flux and
spectrum evolution can be key inputs in improving the knowledge of neutrino
mass and mixing as well as reactor nuclear physics and searching for physics
beyond the standard model. In this work, the evolution of the flux and spectrum
as a function of the reactor isotopic content is reported in terms of the
inverse-beta-decay yield at Daya Bay with 1958 days of data and improved
systematic uncertainties. These measurements are compared with two signature
model predictions: the Huber-Mueller model based on the conversion method and
the SM2018 model based on the summation method. The measured average flux and
spectrum, as well as their evolution with the 239Pu fraction, are inconsistent
with the predictions of the Huber-Mueller model. In contrast, the SM2018 model
is shown to agree with the average flux and its evolution but fails to describe
the energy spectrum. Altering the predicted IBD spectrum from 239Pu does not
improve the agreement with the measurement for either model. The models can be
brought into better agreement with the measurements if either the predicted
spectrum due to 235U is changed or the predicted 235U, 238U, 239Pu, and 241Pu
spectra are changed in equal measure.
