I don't know the correct terms, but the line thickness may not be exported directly to a SVG. It seems to be your case, where the 5 mm thickness are exported as "no thickness" lines.
There is an option in CorelDRAW:
1) Select your geometry and click Ungroup all Objects;
2) Object Menu / Convert Outline to Object (Ctrl+Shift+Q)
It will ungroup all lines and will convert your visible lines to a "real" geometry ready to be exported.
Here is the result in Fusion:

In this another example, i created two circles with different thickness (mm):

And the result in Fusion, applying or not that conversion:

Leonardo Brunelli do Nascimento
Chemical Engineer