I've just seen this reply from Stagecoach NE on X.

We are planning allocating electric buses to services 16, E1, E2 and E6. However depending on servicing we cannot guarantee every bus will be an electric bus.
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Looks like its confirmed they'll go on the 16 too.

One Yutongs is on the X24 today, Bustimes has it allocated till end of service (20:09pm from Newcastle)