Effect of temperature in the free radical polymerization of acrylic acid
Yes, this is expected.

With higher temperature, there is a larger concentration of radicals in the polymerization mixture, and initially the polymerization goes faster than at lower temperatures.  This leads to a faster increase in viscosity, which slows the ability of molecules to diffuse in solution.  This means it is more difficult for unreacted monomer to find growing radicals, and for unreacted initiator to find monomer.  Radicals are highly reactive so when they can't react the have a habit of engaging in termination reactions which are nonproductive regarding conversion.

This is normally called autoacceleration, and is fairly well described in Odian's 'Principles of Polymerization'.  Generally, you do not need to heat radical polymerizations beyond the bare minimum to decompose your thermal initiator.