The consensus over the years is that these 'transitions' from one age to another are buggy. Here are threads I found on the topic:

http://caesar3.heavengames.com/cgi-bin/caeforumscgi/display.cgi?action=ct&f=19,6811,0,all

http://caesar3.heavengames.com/cgi-bin/caeforumscgi/display.cgi?action=ct&f=19,7076,0,all

I'll look for more soon.

I made a little test PAK for testing.
city 0 = distant, buying bronze and bronzeware, selling bean curd and salt.
city 1 = Mission 1(600BCE) & 3(580BCE) player cities, produces bronze, bronzeware, weapons + other stuff, set resources just for this setup(no iron).
both above are set as active/visible & there are no event scripting in the 3 missions.
city 2 = Mission 2(590BCE), inactive/hidden, added iron as a resource and iron being sold.

M1 = City produced the expected stuff(bronze weaponsmith).
M2 = Bronze smelter and Iron smelter co-existed, Iron Weaponsmith was the title after a few game ticks of time. Iron was on the commodity list.
M3 = Iron disappears from commodity list. Bronze Weaponsmith continued to operate.

Next test I will change the date only to 70CE and 900CE.

years 70,80,90 same result as above
years 900,910,920 same result, ran each mission 3 years.
made city 2 visible same result, ran each mission 4~6 months.

made city 2 active: Mission 1, 2 & 3 commodity list shows iron. The now 'Iron Weaponsmith' in Mission 1 and 3 requires player to import iron. Using the 900CE~920CE start dates, ran each mission 4~6 months.
.
Conclusions? or more tests?

[This message has been edited by user3 (edited 10-18-2017 @ 02:30 AM).]

What city was mission 3 at? I can't tell from what you wrote.

@ Gweilo, Mission 3 is City#1 a continuation of Mission 1.

Next test I added an event in mission 2, City#2 becomes inactive, mission complete trigger time.

In Mission 1, I placed the city#2 Trade Station then deleted it after verifying its trade commodities.

Mission 3 result
Iron commodity no longer listed.
Iron Weaponsmith becomes Bronze Weaponsmith.
Trade Station to city#2 no longer available.

Next test will be to place and leave the trade station to city#2.

Results, Trade station drops iron from its buyable produces. Otherwise same as without the Trade Station.
Mission 3 result
Iron commodity no longer listed.
Iron Weaponsmith becomes Bronze Weaponsmith.
Trade Station to city#2 drops iron.

Conclusions, please?

[This message has been edited by user3 (edited 10-18-2017 @ 08:41 AM).]

My conclusion is the same as I've always thought - it's very buggy. Having a city switch back and forth between bronze and iron even though you have progressed the timeline further is just plain silly. Somebody on the dev team failed in their testing.

Nevertheless Gweilo, it is a consistent oddity, thus I would not use the terms "buggy" or "glitch". A designer can use it or at least come to understand it.

This example could be a stepping stone to understanding the other designer triggered transitions.
bronzeware->lacquerware
iron->steel
wood->paper

I added a Campaign Download(539 when approved) of the PAK & Campaign folder(set file) for those wishing to continue the experiment/discussion.
The change I made was City#2 sells Steel rather then Iron now. Read the download page for more info when available.

[This message has been edited by user3 (edited 10-18-2017 @ 12:38 PM).]