You can consider switching to the hallon package.
It does both subfigures and and figure numbering per chapter well.
#import "@preview/hallon:0.1.3" as hallon: subfigure
#show: hallon.style-figures.with(heading-levels: 1)
= Chapter 1
#let example-fig = rect(fill: aqua)
#figure(
grid(
columns: 2,
subfigure(
example-fig,
caption: [foo],
label: <subfig1-foo>,
),
subfigure(
example-fig,
caption: [bar],
label: <subfig1-bar>,
),
),
gap: 1em,
caption: lorem(5),
)