Yes but assuming the decision was also made on 21st
The confusion comes from poor understanding on the requirements and the concept of section 3C. If you are eligible on
the day of decision, you will be granted visa. The idea that HO somehow ignores that you completed your lawful stay under Section 3C and strictly sticks to the date of the application is completely incorrect.
I suggest to read this part of the guide too:
Time the applicant has spent in the UK with 3C leave also counts towards lawful residence.
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Both 3C and 3D leave count as existing leave to enter or remain in the UK, and therefore as lawful residence for the purpose of the 10 year long residence rule.
Also remember in a similar fashion, UKVI procedures allow you to vary a pending application to SET(LR). The date of such application remains the original date of the first application. If HO would stick to the original date as per procedures, anyone who varies his/her application to SET(LR) will have to be refused. This is obviously NOT the case because the date of the decision trumps the day of the application when it comes to calculating the lawful residence period

Advice is given based on my personal research and experience only. Do NOT contact me via private message for immigration advice