I would try OpenExr from Gaea, it's fast becoming the go-to format for heightfield export and is much better than .tif
I've all but retired the .ter format (it's 16bit anyway) it did have the advantage of correctly scaling in TG but you only need a couple of nodes to correctly scale the .exrs
A workaround that Matt suggested is
not to have the entire terrain visible in the viewport when you load the heightfield (i.e. from a high vantage point) - sounds weird but it does work
I just loaded the heightfield with the default camera position, then it could be moved to wherever. Not exhaustively tested though, though I did try this method with my script for loading multi-tiled output form Gaea and it did work when leaving the cam at default position and loading 16 heightfields from a clip file.
Don't know if you gathered the project just to test or to send it off to render?
I wonder if you save the file successfully and then try a render from the command line if that would work?