My new (2nd hand) 590.

[litesport]

I don't know - but I would turn off all other maps to start off with. There is always an issue if two different maps cover the same area. Your OSM maps are probably a different area, but It is still worth trying.

Turning off the other maps did the trick. I had not other maps installed that covered that area, to the best of my knowledge. Anyway, all good.
Thank you for the help.

[jfheath]

That's good. Glad it was one of those suggestions. I didn't know - I have never seen that loss of detail before. But I have seen it routing across places that have no roads when two maps for the same area are turned on, and some other weird behaviour.

Some of the 'maps' are not navigable and they may be turned on while OSM maps are being used.

For example, the one that has DEM in its name is the digital elevation mapping, allowing 3D shading to be drawn.

[raza]

All enabled maps are at least partially loaded into the device's cache when the device is started, and if this data is too much, it simply cannot work, and as a result, no details are plotted and no calculations are performed.