Indeed, the published material ends with tier 5 as far as the cartography is concerned in b4 if
you are looking at the common PDF! You are correct, the last of the room descriptions do not end in tier 5. Several paragraphs are included in b4 catering to the creative talents of the 'expert DM' and furthermore Tom Moldvay outlined numbered room descriptions of the monsters and room specifics per area for the dungeons leading from 6 to 10. Including room 100 the Lair of Zargon. ( I assure you all I share no resemblance...) And then beyond to the Underground City. (although I'm not sure why it's THE underground city, as there seems to be many of these terrible places - haha....)
Rumor has it many physical copies that have been in the possession of a single owner do not have the maps either. Yet other owners maintain their copy came with an unnumbered B/W fold out between pages 14-15. This suggests a few things; more than one printing, mass subterfuge, or theft of the fold out before sale. Alas, the lost maps of the lost city are rumored to be covered in Dragon 315, and 142, and available from Paizo... I can't validate any of that however, but I bet it is worth looking into... What I can, and will do is give you this handy link to a digital recreation!
Enjoy, for what it's worth. I hope you'll find what you're looking for.http://www.users.waitrose.com/~alanderekjones/ylaruam-b4.html