Then the next thing I would suggest (to make sure there is no compatibility issue and/or texture issue) is to create a fresh spline blueprint, and utilize the mesh in that. See if the same thing happens in a simple spline function, instead of going through the Landscape functionality.
There are reports that the Landscapes in UE4+ and UE5+ don’t always play nice, but this may or may not apply to you.
Also, make sure your scalability settings are high enough. They may not be, but, again, I have no idea what you’ve done to this point (you can set them all to “Epic”, just to be on the safe side):
