QGIS: Filtering aggregate returns different results with same value

I have a joined layer that was created from a one-to-many join between a vector layer with three attributes (lat, long, siteid) and a data layer that contains many columns with many entries possessing identical ‘siteid’ fields. I need to find the minimum and maximum range of a field called ‘sampleno’ for each group of entries containing the same ‘siteid’.

Here’s what I tried:

maximum(sampleno,'', siteid=attribute($currentfeature,'siteid'))
|| 'n' ||
maximum(sampleno,'',siteid='1750')

I also tried:

 maximum(sampleno,'', siteid="siteid"))
 || 'n' ||
 maximum(sampleno,'',siteid='1750')

I don’t understand why entering a test value of ‘1750’ as the filter returns the result I expect but a value of "siteid" or attribute($currentfeature,’siteid’) doesn’t.