You are going on the 100 year it seems. Have they updated their Register since March? A quick google isn't pulling anything up, nor a search of their Facebook page, so would mean opening every Whats New? weekly blog after
https://www.findmypast.com/blog/new/1939-worthing-burialsThe info on TNA suggests that death matching was done, but was that just pre-release for the period 1992-2006?
After 2006 they have no English GRO death index entries on their site [dataset runs to 2007 but eg there are only 233 Smiths vs >7500 for 2006/5], same cut-off on Ancestry. After that Findmypast just has the partial Greypower indices 'England & Wales Deaths 2007-2021' online, which Ancestry suggests may be about 55% of them.
The GRO's own online index does not have DOB's. So whilst they open records for people alive or dead transcribed with a DOB of now over 100, but currently redacted from view, I'm unclear how/if post 2006/7 death matching is done. I know GRO deaths are available on microfiche in a half dozen libraries, perhaps they have access to that data too? By 'notification of a death' and the delays I think NAI mean after Death Cert submission to FindMyPast / FOI form.
Findmypast have checked the names and dates of birth in the 1939 Register against the General Register Office indexes of deaths in England and Wales and opened many records where exact matches are found. Furthermore, the records of individuals born 100 years ago (according to the date of birth shown in the Register) are opened at Findmypast.co.uk on a regular basis.
The version of the 1939 Register on Ancestry.co.uk is updated less frequently. Ancestry open previously closed records once a year, on average, and there is sometimes more than a year’s lag between the notification of a death and the opening of a record on their website.
https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/1939-register/The Register was updated until 1991, meaning that anyone who was born less than 100 years and a day ago but died prior to 1991 will have their record opened automatically. If the person died after 1991, we can accept scans of death certificates...
https://www.findmypast.co.uk/1939register/1939-register-getting-started