Since Regular and Medium have the same internal name ("Interface"), the second one installed just overwrites the first. The same applies to the italic variants.
Same issue exists on Windows 10. Following the installation documentation results in a warning dialog asking me to overwrite; once for "Interface" once for the italics.
Hm, this seems like a bug. I know what the issue is and I'll fix it tonight after work.
Should be fixed in 1.2 https://github.com/rsms/interface/releases/tag/v1.2
@y6nH @joestrouth1 Can you please verify that release 1.2 works for you?
1.2 resolves the issue for me (running Windows 10)
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1.2 resolves the issue for me (running Windows 10)