A degree and a half variation over or under 99.5 is perfectly normal, even within the same incubator let alone over time, you just don’t want it sitting indefinitely at 98 or 102.  I rotate egg position within the incubator to minimize effects on hatching window, depending on design this could mean inner to outer ring, left to right or up to down, so eggs develop at roughly the same rate and hatch within 36 hours of one another.  This idea that the climate (or in this case a micro climate) should always stay static is ridiculous, only a vacuum is constant and that means no life, no energy, no movement, no change. The goal is relative stability, mild fluctuations around an ideal number, and while temp spikes or crashes can affect hatch rates unless extreme they won’t completely destroy a hatch.  Life and its intrinsic functions are a constant cycle and flux, whether that is temp, energy, oxygen, or any other variable, if it isn’t, the critter is dead.