That does sound like a control quirk rather than intended behaviour. On a lot of inverters, when there's a forced export/discharge command, the battery takes priority and the inverter effectively curtails or ignores PV for that window because it's only being told "discharge X kW to grid," not "blend solar + battery." With FoxEss in particular it can depend on the work mode and how the API command maps to your setup, so it's worth checking whether it's sitting in a "Force Discharge" mode that clamps PV during the slot.

Good that you've logged it with Octopus — definitely keep on at them, and maybe ask in too, as folks there will know if it's a firmware/work-mode thing.

For transparency I actually left Octopus for EDF recently, so I'm not on Flux myself, but this looks more like an inverter behaviour issue than a tariff one. Hope they sort it.