Looks to me like probably the wrong field is being output in Solo.out/Gene/summary.csv

Same input.
STAR-2.7.9a:

Reads Mapped to Gene: Unique+Multipe Gene,0.484805
Reads Mapped to Gene: Unique Gene,0.453594

STAR-2.7.10a_alpha_220314

Reads Mapped to Gene: Unique+Multiple Gene,0.453594
Reads Mapped to Gene: Unique Gene,0.453594