On the HMRC service availability page, it currently says:
"Employment Allowance and other credits are not visible, or are incorrect on the monthly statement
We are aware that for a very small number of users, Employment Allowance and other credits may not be shown, or may show the incorrect amount on the monthly statement."
Given that at least 4 of my clients are affected (I've only checked 4, but they all had the same issue), it is unlikely to be a small number of users.
I assume everybody else is having the same issue - please confirm and I'll raise it further.