I think this is going to happen to me too. I'm about to change jobs and they use a different voucher provider so I think I'm going to end up as a 'new' scheme member rather than an existing one. I'm waiting t hear from HR.
Joined before 6 April, hence entitled to the max amount even as high rate tax payer. Was made redundant. Got new job. But as it is a new job I now count as new entrant to scheme and my entitlement is half. But my childcare costs haven't gone down.
So I've just learned that this happens to anyone who changes jobs.
It's annoying that changing jobs puts you into the new category when staying put wouldn't. It's like loads on new government policy though; there are very obvious problems and unfairnesses but they're ignored because it's easier to do so.
As it is, I may be fine because the salary sacrifice on my pension contributions takes me below the HRT threshold. My new salary is only a tiny bit over the threshold.