Hi FrodoOfBagEnd - Welcome to the forums (I also love your username!)
Thank you for sharing the details of your experience and for your support while we work on this issue. This issue is occuring in single player and multiplayer so it is likely not related to your partner being kicked.
I have a known issues post for this started here: [Known Issue][PS4/XB1] Loss of interactions
While there are no workarounds, some players have mentioned the issue occuring in a cycle, so the interactions may come back and then go again. Though I am glad you were able to save at your shelter even when this issue was happening - if you’ve played more since your post, have you noticed a cycle of the issue occurring?
The interactions also seem to be related to the water level, you may also find that some items at or below the water - level will still have their prompts accessible.
Some further questions…
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When the issue triggered did you notice any unusual sounds? (some players have mentioned bats without seeing them)
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Do you have any farming plots? If so, are any of them built recently in your safe?
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Do you recall what time of day you started to notice the issue at?
If there’s anything else you can think of, such as items crafted or if you harvested anything when preparing to leave the island please let me know.
Thanks again 