I can reproduce on Xperia 10 II running 4.3.0 as well. However, the video recording also stopped when the screen went sleep which if I understood correctly shouldn’t have happened?
This is a bit special situation: while lockscreen is active, only application that actually has a window (on top of lockscreen) can prevent blanking. And due to sandboxing there was false negative from “topmost window belongs to application requesting blank prevent” check.