So I have only just started playing since frontiers came out and I have base items disappearing. Namely 2 solar panels and my weapons terminal. With the weapons terminal I had the mission to find the Vykeen but that has gone from my log too and the gek overseer wants me to build the terminal again. Also I am noticing when first arriving at my base my solar panels and batteries that remain take a few minutes to re appear.
Welcome to the forum!
This could be multiple issues. Fading in of base parts happens. They used to pop in but Frontiers brought in the fade. Depending on what you are playing and the size of your base could make this more apparent. I am on pc and my largest base takes a few seconds to completely appear.
As for missions disappearing and needing to be redone, first, make sure you are saving and wait a few seconds before exiting the game. Otherwise, this could be a new bug and should be reported https://hellogames.zendesk.com
The two solar panels and wires never reappeared. The rest pop in after reaching the base. The weapons terminal and mission were there at the start of my current play and is now gone. My base is one cylindrical room and a couple of exocraft bays for the Collosus and Roamer. All I have done this session is resource gathering on this planet in the roamer and off world a few times to spend nanites on blue prints. I am on PS4 standard frontiers with all patches the most recent was yesterday 3.65
I have sent a bug report off to that link