Minecraft is saying that I need to buy it again to gain access to full version (I can play...

I have done everything step by step, but it still says "Play demo". Looks like my Microsoft account isn't connected with Minecraft account anymore, but why?

I'm Samy an independent advisor. Welcome to the Xbox community. How are you?

These steps can help you, give it a try.
Check for updates on Windows Update, update everything including optional/additional updates.
1. Logout from Xbox app and Microsoft Store app
2. Look for the Xbox app and Microsoft Store app in the Windows menu, right-click, click app settings and then click terminate, restore and reset. Do this with the launcher too.
3. Clear the store cache type ''wsreset'' in the Windows bar, run as admin, and wait for the screen to close and open the store automatically.
4. Open the Microsoft Store app, click on the 3 dots in the upper right corner>downloads updates>get updates and update everything.
5. Restart your PC, log in to both apps and see if that solves the problem.

Also, try deleting and redownload the launcher from the Xbox app or from the Microsoft Store app on your Windows. When logging back into the launcher select the option to log in with your Microsoft account.

Because it is downloaded in full.

But to gain access, you need to buy the license.

Hello AP4RK,

Unfortunately, you will only able to gain access to the trials since that is what is tied to your Microsoft account, not the full versions. I found this out the hard way when I wanted to buy the full version of Marble Blast Ultra, but it was de-listed. Have
a great day.

There are actually two versions of the deck packs. The free ones are just "compatibility" type packs that let you play against these decks online. To gain access to these decks yourself, you'll need to purchase the full versions of the dlc packs.