Title is correct. When my dweller came back from exploring, and I collected his items, which one of them was power armor, as he took it off for sturdy raider armor. Now the power armor is no where to be found.

Don't forget you can sell items from the explorer's inventory simply by clicking on them (even if you're just looking to see the stats). Anything displaying a cap icon with a number below it (10, 500, etc) will be sold when you click "collect." To fix this, simply click the item again to see the icon for the item itself.

I never sold any armor or weapons while I was looking in his inventory, and I had about 15 spaces left over after I collected everything.

Selling items from returning explorers only requires you to click on them once, and there is neither a confirmation nor even a prompt or warning. I know you didn't intend to sell it, but I'd bet a lunchbox that you did mistakenly. If you're 100% positive you didn't (next time an explorer comes back, click some crap items before letting them in to see what I mean), there's a support forum that might yield better results, and you might even find that it's a common occurrence with other players.

I have another bug from a returning dweller. He had 100 items, but when i collected them, i got only 51 items... I'm pretty sure I didn't sold any item by clicking on them...

Subsidiary question : why the carry limit of 100 items for a dweller in the wasteland ? After that, he will not collect the new items he finds, even if they have better stats, that's stupid, he should be able to replace a bad item with a better one...