Duration Mastery recommended even when user cdr is at 27%

[CanOrhan]

Hey - I'm still new to Ward Loops so don't know if this is actually a bug but I was notified that I should allocation 10% less skill effect duration even though I've hit the 27% CDR breakpoint.
https://discord.com/channels/852104066401173514/988474586490368110/1252400594350768210

Is this the expected behaviour?