As 1st January was my reporting date I went to Rayong Imm. today. The officer said "21st February finished?", with which I agreed as that is the expiry date of my current extension. Only afterwards I saw that the next reporting date given on the chitty is 21st Feb., thus not 90 days. 

I don't recall that from previous years, nor have I noticed if it's been mentioned on this forum. I just wonder if this is now the norm, either locally or elsewhere.

It's not a problem, as far as I can see. In practice it just means that to do both matters on one trip I don't apply for new extension before 14th. Feb. I suppose it's too much to ask that the extension grant generates a report and a new 90-day date - perhaps they're rewriting the software to take care of this......

Some offices do it that way.

My local office did it that way until a couple of years ago but now do it to the 90th day now.

My office has done it that way for years.  No separate lines, the officer that does the extension puts a new 90 day report slip in my passport at the same time.

Khon Kaen office always give you a 90 day date of when your extension expires.  Otherwise it could be construed as giving you permission to stay until the 90 day date rather than the extension expiry date.

They are also slightly different in that when you have obtained your new extension, you have to go and copy that page in your passport  to support  your 90 day application.

But now they have a separate window just for 90 days so I've no idea what will happen next September when my extension expires.

Unfortunately this does not appear to work at Rayong Immigration if you have re-entered Thailand from a trip abroad within 90 days of applying for your next extension of stay, as I have done for the past few years. In other words your next reporting date is still 90 days from your re-entry into Thailand rather than from the date of your subsequent extension application, based on my experience.