Strange behavior of voltage divider
Without knowing your load circuit, I can still say it's most likely an impedance problem. You have two options. Either you can decrease the output impedance, or increase the load impedance. We can't help you with the latter unless you give us more information.

To decrease the output impedance, you could replace your voltage divider with a linear regulator like the CUI P7805-Q12-S2-S, or keep the voltage divider and add an op-amp configured for non-inverting buffer mode.