I can verify this. I have a Oneplus 5 and have trouble staying on LTE. In practice, the issue is less predictable--for example, I've been able to reconnect to LTE after taking a call (though it definitely does knock you off LTE when a call comes in), and I've been kicked off LTE without getting or placing a call. Sometimes I get kicked off of LTE within minutes, and other times I can go hours before it happens. Now that I know to look for it, it's relatively easy to see that I'm not on 4G/LTE and toggle airplane mode on and off to get on, but it is a pain in the ass and something I feel I shouldn't have to do, given how many hoops I've already jumped through just to get on LTE in the first place.
It doesn't help that the 3G/HSPDA network is now so bad that about half the time, if I'm kicked down to 3G/3.5G, it's effectively the same as having no connection at all.