Land Mode Using 2 GPS Blend

I have done a test flight using two blended GPS indoors, the result is that the drone can be in poshold mode but still shifts on its own (like a GPS glitch but no GPS glitch massage). Then in land mode, the drone’s position is slightly tilted, which is unusual when I use 1 GPS. What is the reason?

Poor GPS signal quality indoors might be the issue.
Conduct some tests flights outdoors in a open area with full unobstructed sky view to test it.
If it works then, you will know what is wrong, if it does not you will need to dig deeper.