I cannot reproduce this behavior.
On Windows 7, FileMaker 12.04, Duplicated the Bug Report layout from My Known Bugs List.
I modified the portal for FMP Versions in two ways:
Reduced the number of rows (so that I could get more records than rows in the portal)
Turned the drop down list formatted field into a button using the Go To Related Records button option specifying a layout based on FMP Versions and the Match Current Record Only options.
I performed a find for FMP 9 in this portal and flipped through the found set to find a report with more versions listed than can all show in the reduced size portal.
I then clicked in the portal to put the focus on one portal row without clicking any buttons and used the scroll wheel to scroll the portal.
I then clicked the "button" field of a different portal row.
The system changed layouts and made the clicked portal row record the current record as expected.
I repeated this several times and each time got the expected current record when I clicked the button.
Can you spot what I did differently?
Can you reproduce this behavior on a different, brand new test file?
Getting the first record is a common result if something causes the clicked portal row to "lose" the focus. (commit records is a frequent culprit.)
I seem to also recall an issue when the target table is more than one table occurrence "away" from the layout's table occurrence, but I can't find the report for that in my database.