The Automatically Remove Loops feature is very useful for reworking tracks (traces) by simply routing over the top and allowing the system to rip up the redundant tracks.
This feature is enabled by default in Preferences:
File >> Preferences >> PCB Editor >> Interactive Routing:
Automatically Remove Loops
Sometimes if extra vias and tracks are placed deliberately, for example that is commonly done with power nets, the extra copper is required to be retained.
Rather than enabling/disabling the entire feature, just disable Remove Loops for specific Nets:
Remove Loops is enabled for the GND net. Un-tick this box to disable the loop removing feature for GND.
In the PCB panel, choose Nets in the drop down, then click <All Nets> and click the Net such as GND.
Double click to show the dialog for that net.
Un-tick Remove Loops.
Note: Unlike the global setting in Preferences, the Net settings are saved into this board only.