The bug
After flying about and exploring the world for about 8-9 minutes I noticed 2 things. 1) Lag spikes randomly occurred as I was loading new chunks. Not too surprised by this as I have seem this on full versions. 2) After about 7 minutes of just flying in one direction I began experiencing numerous lag spikes, one after the other, quite frequently. I tried it 3 times in three different directions, and it doesn't appear to matter what biome you're in or anything. F3 charts are below.
After performing a second test I noticed a few more things. Reloading textures fixes the frequent spikes, and if you are using the super fast fly in Spectator it happens much quicker and is much worse. Also noted that it actually appears to be the random lag spikes that are getting more and more frequent.
Did yet another test and found that entering the menu appears to stop it. I also recorded this test so here's the link: https://www.youtube.com/watch?v=78HEKgEXH9U
Linked issues
duplicates 1
is duplicated by 1
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This is a memory issue.
When the game hits almost 100% memory usage of the memory allocated, the game freezes and clears RAM to prevent the game from crashing like it did in older versions.
Also in Minecraft Java Edition 1.12.2.