I have the same problem, very annoying!!
There is no way for this on Edge 130. If you zoom in, the device will always reset the zoom level to its default. I sold mine Edge 130 because of that, and bought Edge 530.
Did you switch off Auto Zoom? (settings Main-Map-Auto Zoom)
That does not help for this problem, the 130 always returns to 300m, regardless of the AutoZoom setting.I find the 300m setting just right for the map screen, but that's just me.