The 1.16.0 version is causing chunk errors in older chunks.

Not sure exactly what's causing it[1], but it appears to be happening to any chunks that haven't been loaded in a few updates.

[2] is of chunks that were not loaded since sometime in 2017 or 2018, surrounded by chunks last loaded in September 2019. The 2017 chunks have completely disappeared. However, the mobs were still there (and fell into the void)

I tried using an external program (MCC Tool Chest PE) to check the chunks, and in an unloaded backup of the world the chunks appear completely intact.

I also tried opening an old superflat world I created in 2014, but last loaded in 2017 also.

As you can see on [3], the chunks were also completely erased.

I tried, with a backup of this world, to convert the world using MCC Tool Chest into Java, then back to Bedrock. Interestingly, this actually fixed the problem of the chunks disappearing, as shown in [4]. This should hopefully prove that the chunk data itself was intact and not corrupt.

I also had an issue with worlds that hadn't been loaded since 2014. I tried opening a world from this time in the last version (1.14.60 I believe) and whilst it was slightly buggy, the world worked completely as intended once I converted the world to infinite.

Trying the same world in the current version, however, the world took around 10 minutes to load, and when it did, the entire world was completely empty. No blocks at all. Trying to convert the world to infinite crashed the game after a couple of seconds. Checking the files, the game renamed the existing world to [world name]-Old and created a new folder called [world name], then crashed. The new folder is completely empty.

I reinstalled the game to see if that would solve the problem, but no avail. Interestingly, reinstalling the game does seem to have made the world that hasn't been loaded since 2014 load much faster, but I still had the same error as before.

Edit: attached an image of the fully empty world from 2014 (Minecraft 24_06_2020 01_37_12.png)

Edit 2: I've managed to verify that it's a problem with a recent-ish update. I have an old system with Bedrock 1.12 on it and all old worlds loaded perfectly fine in that version.

[1] More information on the possible cause is at MCPE-87376
[2] Minecraft 24_06_2020 01_07_07.png
[3] Minecraft 24_06_2020 01_17_29.png
[4] Minecraft 24_06_2020 01_18_57.png