If you have root access, execute command I'm showing in your root shell. Else, if you don't have a root shell, consider plugging device via USB and access ADB shell. Termux full screen doesn't seem to be working well. Because, they need to add immersive option to source code of Termux. You could fix it be decompiling the apk and add the immersive feature (2lines of code which you can find in Android doc) and recompile.
You can hide;
1) Navigation bar 2) Status bar 3) Fully immersive
•immersive.full: hides the status bar on top of the screen and the navigation bar on the bottom.
settings put global policy_control immersive.full=com.pkg.name
•immersive.status: hides only the status bar on top.
settings put global policy_control immersive.full=com.pkg.name
immersive.navigation: hides only the navigation bar on the bottom.
settings put global policy_control immersive.full=com.pkg.name
Note: If you're using ADB , add ADB shell at beginning of command
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