It’s actually normal that red ribbon shrub lose leaves sometimes both green and red. The unstable temperature during spring is identified to be the cause which can be very hot during the day and very cold (even frost) during the night. Pruning it would help grow new shoots and you’ll have red leaves again.
From the shrub hedge my neighbor has, I saw that those bright red leaves slowly turn green, it’s perfectly natural. Only the young leaves are red. Then they turn green as temperatures heat up. I think it’s connected to frost and such, when they’re young it’s early spring and keeping the green chlorophil for later protects it from freezing.