Found a new issue with conhost in Windows 10 20H1/2004 as pushed to the release preview ring.
Windows 10 build 19041.208 (up to date release preview ring) on a Surface Book 2 15", conhost.exe file version 10.0.19041.153.

The following code, compiled as CUI and launched from a cmd.exe prompt, will crash conhost, closing the window.

int main()
{
HANDLE hStdOut = GetStdHandle(STD_OUTPUT_HANDLE);
SMALL_RECT srNewWinSize;
srNewWinSize.Top = 1;
srNewWinSize.Left = 1;
srNewWinSize.Right = 40;
srNewWinSize.Bottom = 20;
SetConsoleWindowInfo(hStdOut, TRUE, &srNewWinSize);
}

This used to work fine in previous versions of Windows 10, it seems to be a new issue with 20H1/2004.