Week() function

Manfjon67, I would not expect this to be a change caused by newer cf versions (let alone a bug, your current assumption). Instead, I suspect this is more about a difference at the Java level between your cf7 and cf2018, or perhaps the os level if they are on different machines.

First, let me say also that the setlocale you're doing would only have affected locale-specific functions, like lsdateformat vs dateformat. There's none for week(). (And some functions take a locale as an arg, but week does not, though dayofweek does.) 

So instead in your case again I expect this is caused by and can be fixed by jvm or os settings instead. As you may know, since cf6 cf runs atop Java. And java version differences or Java settings can account for what seem differences between one cf implementation and another.

In this case, the day of week calculations are indeed affected by Java, as discussed in places like this: https://www.oracle.com/technical-resources/articles/javase/locale.html

That discusses locale (country or region) settings, which can be set as system properties the jvm startup args. In cf, those can be set or changed in the cf admin "Java and jvm" page or the corresponding jvm.config file that updates. 

You should assess that Java args line there, on your cf7 and cf2018, to see what values if any are there, and to consider changing the 2018 one to be similar to the cf7 one. Or if the 7 one had none, certainly that was a much older jvm so may have had different defaults. (Or the OS on which it was installed may have had different defaults, which the jvm inherited.) 

Btw, if these two cf instances where on the same machine, one could wonder, "couldn't they both be using the same jvm"? Not by default, as each cf instance implements it's own jvm. And regardless, each cf instance sets its own jvm args affecting its startup.) 

Let us know if this gets you going.  I'm writing from my phone and can't test/prove it, but wanted to give you something to try. 

Indeed, that doesn't look good. The week number is more or less universal. You might want to report a bug.