I read from a few sources (reddit, steam discussions) that putting down a land claim block in a POI prevents that POI from resetting from a trader quest. Is this actually true? If so, can a dev confirm?
Yes it is. If there is a sleeping bag or a lcb inside the poi-area, you can't start the quest, so it does not reset. Instead you get a message, that the quest can not be started, because there is a lcb or sleeping bag inside.
You can easily test this yourself. Take a random quest, place an lcb at the poi, try to start the quest.
Be aware though that if you are on a multiplayer server that greatly exceeds the supported player count, and are using multiple LCB's, it is known to bug out sometimes. One of the many reasons to avoid using a POI for a long-term base.
I asked cause back in A17 or A18 i had a dedicated server me and 1 other person played on. We built a base in a POI, had sleeping bags down along with a claim block and the POI still reset when a quest was accepted at that POI.