Hey essem I don't know if you saw my other message cause it was an old thread, but

• I have some issues I was wondering if you could help me with. If you could help me out I would really appreciate it! I might not see your response on this website, but if you could please add me on discord - CsH#7612 or send me your discord I would really appreciate it! I mean if you don't have discord that's fine, but could you reply to this message so I know? Thanks again
• CsH

Thanks for digging the code @Black-Hole
Hope the issue creator will update the issue to reflect the new information and/or a contributor will look into it.

On a fork of Spigot I found the explanation that item entities are checking every 4th tick if they should fall. Combined with the entity activation range this could lead to some items to never fall on the server. The releavant line in EntityItem looks like this:
if (!this.onGround || b(this.getMot()) > 9.999999747378752E-6D || (this.ticksLived + this.getId()) % 4 == 0) {

Hello,
i acutally did some tests on this issue after experiencing the same (and a similar case). It dosn't seem to be related to the Vanilla-issue linked by @Black-Hole since it dosn't appear in vanilla.
I used the same (wooden-)butten, redstone torch and piston setup for all tests:

Vanilla Singleplayer: Everything works fine, the issue dosn't occur.
Vanilla Server: Same as singleplayer, no issues.
Spigot Server (2 different ones, no plugins/datapacks to be sure): About 25% (differs) of the dropped items teleport back to the original positon

The hitbox moves clientside with the item (F3+B). The 'teleporting' stacks are real -> can be picked up and used. Affected are item groups, not invidual items in a stack (but also single items if they are not part of a stack)

Now to the other probably related issue: Leave decay drops. Some of the items appear to be on the ground but can't be picked up, only if you get somewhere near were they dropped they teleport back while picking up.
Both issues let me think of a problem with items falling after a block beneath them has been removed.

(I've observed this issue now since a while, its quite annoying for chopping trees)

Edit: Used version 1.15.1 (also happened with 1.15 and at least some 1.14 versions) and newest spigot as of writing this.

Might be related to https://bugs.mojang.com/browse/MC-4

I did try in singelplayer and vanilla server, the problem is only happening on spigot and papermc

Please try to reproduce with a vanilla Minecraft server.