That definitely hasn't been by experience since switching to Verizon a few weeks ago. Now that my iPhone 13 Pro can actually connect to mmWave I'm finding it everywhere the Verizon map says I should be, and even a little more. My phone immediately switches to it when available and routinely hits over 3 Gbps. But yea, move a few feet and it vanishes. I have gotten some impressive range, though. eNB 84364 has a new antenna added to a sector recently (Myrtle Ave at Ashland Pl) and I can pick that site up on the other side of Flatbush Ave at 3+ Gbps.
Just like the map says, I can walk down Myrtle Ave in Fort Greene between about Washington Park down to about Pratt and my iPhone stays on mmWave. Ironically, though, the iPhone seems to hold on to it even when the speeds dip far below what even LTE could provide.