I’m not sure what you are doing differently but I not only lose a ton of units but I also lose the commanders they were packed into as well. My cities get demoted to towns. Relationships soft reset. Unit positions reset. Yield soft reset.
Just very anti-4x design
Wow, that is a big difference between our experiences.
A few more specifics, if it helps --
In Antiquity, I build 2 (maybe 3) army commanders. They survive, packed with infantry and cavalry. The settlement limit is usually 4-5, so each of my towns/cities has a ranged unit defending it; they usually survive. Yes, I've seen that my siege units don't survive. If I built a boat or two, the boats don't survive. In another thread, I read that 6 units survive which are not packed.
In Exploration, I add another 2 army commanders and usually 2 fleet commanders, one in each ocean. I expand up to (and past) the settlement limit with each one defended by a ranged unit. Again, my commanders always survive. Last night, I got a warning as the Exploration crisis started, to build more army commanders or I could lose 6 land units at the end of the age. I don't always prioritize the commander promotions that unlock the extra slots; I probably should. I've been picking the "buff this attacker" promotions while in the midst of a war, to make that war go better. I don't remember whether my siege survive or not, from past games.
Yes, cities in Antiquity get demoted to towns in Exploration, unless I got the correct golden age. Yes, cities in Exploration get demoted to towns. I regularly use the "switch my capital" to start Exploration with 2 cities, old capital and new captial. I usually start the age with enough cash/gold to promote a town that used to be a city back into a city; often I can re-promote 2. I like that I can choose a different specialization for towns in different ages. Farming towns (in Antiquity) might become relgious sites in Exploration, then hub town or factory town in Modern.
My relationships in Exploration have a huge influence from the state in the previous age; grumpy usually stays grumpy. Yes, units get moved around; I'm not a fan. The buildings all remain, but the non-ageless buildings lose nearly all of their adjacencies. By design, they are ripe for overbuilding.
I retain all of the map knowledge; the fog of war doesn't re-hide the tiles I have already seen. I retain all my settlements, roads, and tile improvements.