it seems that you are more expiriented than others. I just tried to explain some problems that can occur with 1:1 relationsships.
1.) and 2.) thats true, and if this is your desired result everything is fine.
3.) the main mistakes made for a 1:1 is that the undestanding that empy values "" and Null values are treated same.
4.) did you cleanse duplicates and then combined 2 Columns? Maybe thats the reason you have duplicates in your Key column in HREF
But I can't use the Query Editor cause I don't have the source files. So I have only Dax Methods to check your data
But neither I have null values in my data, nor duplicates. That's why the topic name contains "bug?"...
4) --> I created new calculated column, concatenating two strings. I removed duplicates after the creation of the column. I also tried both to transformate my data directly in Power BI and outside of it, with Jupyter Notebooks. Same result.