Users receiving sporadic access denied screens to public sites when they have access in permissions

My organisation uses communication sites that are open to all within the organisation - ie. Site visitors is set to 'Everyone Except External Users' with read-only access.

Recently individual users have started receiving "access denied" screens to sites that they could previously access without any issues (including the main organisation wide homepage itself;f)

If I go into check permissions and put their name in it correctly says they already have access via the visitors group.  Accepting the access request adds them individually to the site visitors group and fixes the problem but this is not ideal and the volume of these issues is increasing across multiple sites

Everyone except external users uses this account:

c:0-.f|rolemanager|spo-grid-all-users/936a6dcc-9519-40e4-b81a-5c216b42b6f4


Any ideas on what we could do to investigate / resolve this?

Thanks
Ian