Actually there is a pretty common way that soldiers lose much of their kit.
Yes. There is a general problem with people viewing What Happens In War through the lens of recent experience, and recent experience is not a high-intensity conflict against a peer or near-peer adversary. I agree that soldiers in WWIII Europe likely would have lost most of or all of their kit, & stragglers sneaking out of Kalisz would likely have little more than their weapons and the clothes on their backs. Especially if you travel in vehicles, which tend to burn nicely.
At the same time ... it's weird that everyone has a backpack, if that backpack is empty! If you have a backpack, you ought to have accumulated some stuff in there, leading me back to sleeping bag or coat; rain gear or blanket; shelter half or....
There is another quirk here that no one has mentioned: only soldiers get backpacks. So I can generate a civilian who is lucky enough to have 6 days of food and 6 days of water, but this character is apparently presumed to carry it in their pockets. I think everyone should be presumed to have a day pack or satchel (or the butt pack on their web gear) to stuff with incidentals (such as a blanket or a rain coat or whatever.) It's important not to have a system where military characters are wildly more gear-rich than civvies (except with respect to weapons).