For us in Europe using the ISO standard for week numbering one of the fixes delivered in Notes 8.5.1 FP2 has been a long time coming. Previously when using the calendar regional setting of "Use my operating system regional setting" Notes would still get the week number wrong. Of course the solution was simply to specify "Always use the ISO standard" though it didn't feel right.
Now with Notes 8.5.1 FP2 there is a fix for this issue and Notes will now behave correctly (at least it does so on my client).
"SPR# FPAI7X7KL3 - There is a preference for how week numbering is handled in the Notes calendar. There are 3 choices for the preference: "Use my operating system regional setting", "Always use the ISO standard", and "Custom week numbers". Prior to this fix, the choice of "Use my operating system regional setting" was not working properly for those users who had their OS regional setting set for a country/language that used the ISO standard. It should be noted that this fix does not make an inspection of the registry settings (on windows platforms). Instead, the fix uses the Java JVM's settings for week numbering based on the country/language the user's OS is running in at the time the Notes client is started. "