Yes, immersion can have an effect on some IQ test scores.
How do I know? We speak French at home, but DS8 was in an English only daycare/preschool from 3mo. onward and at 5 was on his way to being receptive only in French. So we enrolled him in a Spanish immersion program for K.
After two summers in France which convinced him he could actually speak French at home and one year of K mostly in Spanish he had a full evaluation at school, which includes a WISC in English. He had an 8 on the vocabulary subtest. Fast forward 5 months, with 2 months of English phonics and daily English reading at home. He was re-tested on the DAS and got the equivalen of a 14 scaled score on word definitions (which is I think equivalent to the WISC vocabulary subtest). 5 months of cramming and we jumped from the 25th to the 90th percentile.
So yeah, there can be an impact, and I'd take results with a grain of salt. Although my son tests so weirdly that... Maybe he is an example of one (weird fact: he was close to the age cut-off so was administered parts of both the preschool and the elementary age batteries of the DAS, and ended up with a score at the 58th percentile for preschool vocabulary and at the 90th percentile for elementary vocabulary).
