It is true that, in general, research does not focus on whether or not service-learning actually promotes learning, because it is often assumed that it does provide learning. Still, there is some research that does point to the fact that service-learning does promote learning.

First, as Gomez-Estern, BM; Arias-Sánchez, S; Macarro, MJM; Romero, MRC; Lozano, VM. (2021) states: “SL students reported significantly more changes connected to a concept of learning that included contents, behaviors, and personal changes”. In other words, students who have received service-learning-based education report a higher level of appraisals and perceived changes than those who have not followed the service-learning program.

On the other hand, Jenkins, S. (2010) states that: “Both service learning and simulations have been shown to positively impact student outcomes”.

In summary, there is not much research on whether service-learning promotes learning, but in those few investigations in which it is analyzed, it seems that they are more inclined to the position that it does promote learning.

References
Gomez-Estern, BM; Arias-Sánchez, S; Macarro, MJM; Romero, MRC; Lozano, VM. (2021). Does service learning make a difference? comparing students’ valuations in service learning and non-service learning teaching of psychology. Studies in higher education, 46 (7), 1395 – 1405.
https://www.webofscience.com/wos/alldb/full-record/WOS:000952115500001

Jenkins, S. (2010). Service Learning and Simulations. PS – Political science and politics, 43 (3), 542-545.
https://www.webofscience.com/wos/alldb/full-record/WOS:000279626600023