Items held in hand (like being in the process of being placed) disappear when switching between above and below ground and vice versa.

They go into your inventory. This is necessary since you may have items in hand that aren’t allowed underground.

No, I mean a full stack of belts will disappear when I switch from underground to above. I get what you are saying, though. I'll see if I can replicate it.

I have the same issue, specifically with belts. It hasn't happened to me with splitters or anything like that. I've definitely lost around 1000 belts, not when transferring from above ground to below, but when finishing below ground and going back up. If I have my transport belts selected, I lose them.

Thanks for the report! This should be fixed in 0.4.7.