Leon Wilk

Attached a test world download. Instructions on signs.
Add or remove items to the lower of two chests (varied items, not more of same type) to make the effect more pronounced or weaker.

Yes, you can; a minecart without items in it will pick them too - after picking the "non-matching" ones first.

This definitely affects Java Edition. My report MC-155433  seems to be duplicate of this in case of hopper minecarts, reported JE 1.14.3, AFAIK still present.

Related: MC-119369  - boats do break into planks and sticks, but only dropped from particular, very specific heights: 12.5, 13.5, 49.5, 51.5, 111.5, 114.5, 198.5 or 202.5 blocks.

Also, confirmed 1.16.3

Notably, the boat breaks into 3 planks and 2 sticks like before 1.3.1. Might be somewhat related to MC-89954  - should boats even break like that at all?

The height appears to correspond to ((5x)²±x)/2

The search distances for matching portals have been shortened - 15 blocks for the Nether,  127 blocks for the overworld. Meanwhile the range of search for viable location for spawning a new portal is unchanged. This frequently leads to newly spawned portals no longer leading back to the origin.

The game seems to search for existing portals in the Nether within 15 blocks (not 127 as it used to) from the point [floor(x/8),y,floor(z/8)]. Failing to find any it goes about finding viable portal spawn locations within a much wider range (127?) and spawns a portal at one found. That portal will likely not link back to the same overworld portal as the new exit point [x*8,y,z*8] will most probably be way more than 127 blocks from the previous portal - no longer does the game search the 1024 blocks radius.

Still present in 20w11a. Used to be that it happens only once the pressure plate is depressed (and then it stayed depressed). Now it applies to both undepressed and depressed pressure plate (one fights with the frame border, the other with the canvas), also applies to the new wood type pressure plates. It used to be a trick to conceal the frame and display the item on decorative surface, but with the z-fighting of the frame border this is no longer viable.

@Th3F4114n0n3: Are you sure the horse released of the rider should just stay immobile on the bottom and shouldn't try heading for the surface?
This happened to me too,

Confirming, happened to me a few times too. The particular circumstances usually involve a moment of lag just when the minecart (with player character inside) is starting (launched from a booster track activated by button, the lag may be of external origin, say, Firefox updating a webpage in the background and simply taking up all CPU power for half a second or so), The minecart starts moving noiselessly, while the sound source remains stuck to the location of start, independent from the minecart's fate or speed. You can hear the noise coming from some point near the origin of the track, it vanes as you go away.

Sometimes the glitch vanishes if you start the minecart anew elsewhere, more often it remains there until restart while minecart movement remains silent. I can't swear by it but I believe it survived the game restart once too.
Repeating is difficult because it depends on the game "hiccuping" (a moment of lag) while riding the cart.