https://www.legislation.gov.uk/ukpga/1990/34/section/2
British Nationality (Hong Kong) Act 1990
Section 2
(2)A British Dependent Territories citizen shall cease to be such a citizen if he becomes, or has before the coming into force of this subsection become, a British citizen by virtue of this Act.
https://www.legislation.gov.uk/uksi/1986/948/article/4/made
The Hong Kong (British Nationality) Order 1986
Article 4
(3) Any person who, having become a British National (Overseas) by virtue of paragraph (2) above, ceases at any time before 1st July 1997 to be a British Dependent Territories citizen shall at the same time cease to be a British National (Overseas).