Probably not a bug, unless this was supposed to be listed in some order.
When same actor appears in the same show multiple times under different roles there don't appear to be any order to it.
Nor by character's name or surname, nor by episode number, nor by episode release date, nor by episode title.
This may be ordered in the way it was added to the database, which probably isn't good :)
I added credits for episode 1x35 few days ago, and just recently for 2x29 which supports my theory that the ordering is done in the way it was added to the database.
In the way it is currently presented, it would at least be better if these are displayed alphabetically (by character's name).
User page: https://www.tvmaze.com/people/138207/neil-hamilton
Excerpt in question is also pasted below.
Perry Mason (1957)
Guest starring as Victor Billings
Episode 8x05: The Case of the Betrayed Bride (Oct 22, 1964)
Guest starring as Henry Noble
Episode 3x07: The Case of the Golden Fraud (Nov 21, 1959)
Guest starring as Bertrand Allred
Episode 1x35: Erle Stanley Gardner's The Case of the Lazy Lover (May 31, 1958)
Guest starring as Frank Livesey
Episode 2x29: Erle Stanley Gardner's The Case of the Dubious Bridegroom (Jun 13, 1959)