SubPageLink and its reference

UlisesSas

Hi, I've encountered what I think is a bug (unless several hours without sleep could be clouding my judgement), though I guess it could be by design.
I've modified the Sales Invoice page to show a subpage in its header, to show a summarization of the Items being invoiced, its quantities and average unit price.
To do this, I added a couple flowfields (one for quantities and one for unit price) to the Item table, and a Flowfilter (for the Invoice number to filter by). I added a SubPageLink between the two pages (the Sales Invoice page and my new subpage) using the "No." field as link.
I wanted to give the user the ability to modify the "Avg. unit price" from the header, but to do that, I had to be able to first, get the right Invoice No., and that's where I found the issue. The fact is, If I execute a "getfilter("Invoice filter")" (that's the flowfilter field) inside my subpage, it will not find anything. I even debugged it to try to find where was I making a mistake, but there is no reference to the Invoice no. in the subpage... and strangely both FlowFields work fine.
Someone knows why is that?

Thanks for any insight!