I currently have visa o (Thai spouse) which expires on 10 March. On 31st December I have reentered Thailand. And immigration only approved my stay until 10 March. I don't know whether this was intentional or a mistake. But in the past years they always gave me 3 months, even when I entered the country on the last day of my visa's validity.
Did it change or is it because I have entered the kingdom with a reentry visa (not multi entry)?
TLDR : Answer Summary
The reason for your stay being granted only until 10 March, the expiration date of your visa, is because you reentered Thailand on a reentry permit instead of a multiple entry visa. When you enter the country with a reentry permit, the duration of stay you receive upon entry aligns with your existing visa extension, which is linked to your marriage to a Thai national.