If muscle length is changing during the contraction, then "yes". If muscle length is not as during an isometric contraction, then "no".

I think most muscle physiology textbooks will discuss this. The following website seems to do a good job as well. https://www.khanacademy.org/science/health-and-medicine/human-anatomy-and-physiology/nervous-system-introduction/v/muscle-stretch-reflex

A paper by Meunier, S., and C. Morin (1989) demonstrated that both homonymous and heteronymous Ia facilitations are markedly increased at the beginning of a voluntary isometric contraction.

Another study by Dietz et al. (1979) showed a decreased presynaptic inhibition of Ia afferent during running.

Therefore it seems that the answer to my question: "Do Ia afferents from muscle spindle contribute to voluntary muscle contractions?"

is, yes. regardless the contraction type

Moreover the contribution is also in relation to heteronymous-mediated decrease od presynaptic inhibition on Ia homonymous (contracting) muscle with symultaneous increase of presynaptic inhibition of antagonist

Meunier, S., and C. Morin. "Changes in presynaptic inhibition of Ia fibres to soleus motoneurones during voluntary dorsiflexion of the foot." Exp Brain Res (1989).

Dietz, V., D. Schmidtbleicher, and J. Noth. "Neuronal mechanisms of human locomotion." journal of Neurophysiology 42.5 (1979): 1212-1222.