David Marshall says of the haiku sonnet:

Formally, it combines four haiku and a final two-line “couplet” consisting of seven syllable and/or five syllable lines, making 14 lines.

Conceptually, it’s an attempt to wed two like and unlike forms. To me, the sonnet seems the quintessential western poetic form, defined by the order and rationality of its problem-resolution organization. Depending how you see it, the haiku might be just as organized—haiku certainly have strong rules and conventions. Because haiku can rely, just as a sonnet does, on a sort of reversal—a “volta” in sonnets, a “kireji” in haiku—they may be distant cousins. However, haiku are eastern, and, where sonnets are rational, haiku are resonant. Where sonnets solve—or attempt to solve—haiku observe.

Helpful links

https://dmarshall58.wordpress.com/haiku-sonnets/

https://adamoftheuniverse.poetry.blog/2019/09/12/week-sixteen-the-haiku-sonnet/

https://www.writersdigest.com/write-better-poetry/haiku-sonnet-poetic-form#:~:text=Haiku%20Sonnets,a%20poet%20named%20David%20Marshall.