Assertion: There is evidence that artists were working in Haripur–Guler ever since the reign of Dalip Singh.
Reason: Many of his and his son Bishan Singh’s statues, dating back to earlier than 1730s, i.e., before the beginning of the Guler–Kangra phase can be found.

The Assertion is correct but the Reason is incorrect.

It's not statues but portraits that have been found.

Guler appears to have a long tradition of paintings amongst all Pahari schools. There is evidence that artists were working in Haripur–Guler ever since the reign of Dalip Singh (1695–1743) as many of his and his son Bishan Singh’s portraits, dating back to earlier than 1730s, i.e., before the beginning of the Guler–Kangra phase can be found. Bishan Singh died during the lifetime of his father Dalip Singh. So, his younger brother Govardhan Chand ascended to the throne that witnessed a change in painting style.