LDraw.org allows third party part in the LDraw Parts Library

[Eugen]

I have no clue how LEOCAD renders this softened edge, there is no stud like this in the library, so it seems to me that this is a LEOCAD function that LEOCAD needs to address.

"there is no stud like this in the library",— it is in the library, see "stud-logo2.dat", "stud-logo3.dat", "stud-logo4.dat" and "stud-logo5.dat" (for softened edges open/hollow studs there are "stud2-logo2.dat", "stud2-logo3.dat", "stud2-logo4.dat" and "stud2-logo5.dat" as well).

And LeoCAD just uses these studs from library to substitute "stud.dat" in normal parts (for Third Party parts only "stud26.dat" is used in all of the cases).

There are only three studs with high-contrast in the library, stud, stud2 and stud20
I don't know how LEOCAD works/is supposed to work, respectively how it replaces studs with the high-contrast versions, you could add a stud26-high-contrast to see if this addressed your issue.

Adding "stud26-high-contrast.dat" has no effect. I will report this issue on LeoCAD issues tracker.

Also found that "High Contrast" studs are not documented in "Primiteves Reference" on Wiki yet: https://wiki.ldraw.org/wiki/Primiteves_Reference
Only "stud-high-contrast.dat" and "stud2-high-contrast.dat" are mentioned in a separate Tutorial page https://wiki.ldraw.org/wiki/Studs_with_Logos
(and all of the "High Contrast" studs also are not in official library yet: https://library.ldraw.org/tracker/29638)

This has to be addressed to the programmer of LeoCAD. It's not that the tail wags the dog.

Yes, it would be reported to LeoCAD developer (and some of LeoCAD's issue related to Third Party parts already reported).

I just highlighted that Third Party parts could be not fully backward compatible with the previosly released third party sotware for LDraw.