Yes, your hand can mask an antenna module (made of several basic antennas, or patches, for beam forming) and make it useless. This is why 5G phone designs have several such antenna modules spread around so that at least one of them should be usable, most of the time. IIRC it's 3 modules for current gen 5G phones. Of course in some cases it will be your body shadowing the lone 5G cell around, and in this case the phone will fall back to 4G (or 5G in lower bands, so not mmWaves).

All those additional antenna modules explain why 5G phones are on the big side.