Using MS's cascadia code font with ligatures enabled, this extension being enabled appears to influence how the ligatures render in .dfy files.
e.g. extension disabled

vs extension enabled

This is on MacOS Catalina 10.15
VS Code 1.39.2
Extension 0.17.1
Cascadia Code 1910.04
(latest versions of everything)
Using MS's cascadia code font with ligatures enabled, this extension being enabled appears to influence how the ligatures render in
.dfyfiles.e.g. extension disabled

vs extension enabled

This is on MacOS Catalina 10.15
VS Code 1.39.2
Extension 0.17.1
Cascadia Code 1910.04
(latest versions of everything)