There is an inconsistency between documentation and typings for breakpointMods.
Typing for breakpointMods suggests justify-content-flex-space-between for modifier which is invalid and undefined. Correct value is justify-content-space-between.
Using justify-content-space-between leads to TypeError:

TS2322: Type 'string' is not assignable to type '"column" | "flex" | "inline-flex" | "column-reverse" | "row" | "row-reverse" | "nowrap" | "wrap" | "wrap-reverse" | "full-width" | "spacer-none" | "spacer-xs" | "spacer-sm" | ... 44 more ... | "align-content-space-around"'.

breakpoint should be optional because docs shows breakpoint not being used with justify-content-space-between.
Documentation for PF4 show the correct implementation for this: https://patternfly-react.surge.sh/patternfly-4/documentation/react/layouts/flex#justify-content-space-between

<Flex breakpointMods={[{ modifier: 'justify-content-space-between' }]} />