An update, just discovered straight from 1.06 testing: I had this happen to a bunch of units outside a town, because I loaded the game from a save at that point.  If you have damaged groups of units, save the game, and the reload, problems are extremely likely.

I had the same problem with a unit in my sovereign's stack outside of a city.  I was playing (if it matters) a custom Empire.  They were damaged so they had two units instead of three, I saved/reloaded, and they were now only a single unit with reduced stats.  

Yeah, I had this happen too in the campaign back in 1.05, had a damaged group of units, and I noticed that they had turned into single units, so I went back to an earlier save, to try and find out when that happened, and kept going back, and found that was the same in all of the saves where they had been damaged in, even the ones where they had initially been fine when I first played through that part.  Any save that I had where they were damaged, they started out as individual units.

 I did not think it was that big of a deal i thought that was supose to happen for example combat some units die so the total health of all units would go down healing dose not bring people who die in that unit back to life and if theres less people then theres less health and units would go down is that not how its supose to happen  I have no had any issues with health potions of magic that i have noticed out of normal ?

Yep, I've noticed this as well - resuming a saved game will turn damaged squads into single units.

Just FYI, prior to 1.06 this seemed to happen both with saved games and any time the grouped unit healed in a town.  It almost seemed like the game was calculating # of troops in the unit as a ratio of current vs. max HP.. so an even slightly damaged unit steps into a fortified town, has it's max HP doubled but not it's current HP and suddenly hemorrhages troops.  Thankfully I've tested several times in 1.06 and haven't been able to reproduce the city version.  The save version is alive and well.  Of course just now my frustration at losing several large units of axemen pales in comparison to the greyed out end turn button that's just killed my 150 turn campaign.