Corona Renderer 1.7 for 3ds Max (Hotfix 4) is out!
Download the new version from https://corona-renderer.com/download
Read what’s new in Autodesk 3ds Max 2019 at https://help.autodesk.com/view/3DSMAX/2019/ENU/?guid=GUID-40222876-7E08-42F8-A912-F6CDC597A3FD
Camera shift bug sorted out in this one? 
yes, it includes all the fixes from previous releases up until 1.7.3
Cool, but.. does that include a fix for the fact that cams saved in 1.7.2 don’t open correctly in 1.7.3/4? See: https://forum.corona-renderer.com/ Corona Renderer 1.7 for 3ds Max (Hotfix 3) released!
“Unfortuantely” everything is working fine here, so we were unable to tell what might be wrong on your end. We have your scene, and we have a task for this created in our tracker. We will definitely get this fixed asap.
Just to add to the testing, when I tried your to open your scene in 1.7.4, I get no Legacy checkbox (opened in Max 2018 3 and 4) as you report.
If I load the scene using Corona 1.7.2 and then save it without making any changes (just saving to a different file name) and then open the resaved version it in 1.7.4, I then do see the Legacy checkbox.
If I load the scene in 1.7.4 and resave it, this resave still does not show the Legacy checkbox on loading - so only if I resave from 1.7.2 does it fix it.
I created a simple test scene in with 1.7.2 and saved it, but it opened fine in 1.7.3 - so I haven’t found any way to duplicate saving a scene where the Legacy checkbox doesn’t work, unfortunately. I did try saving a scene with 1.7.2 and Max 2018 patch 3 and opening in Max 2018 patch 4 , but that didn’t duplicate the issue either.