Apparently there are a lot of bugs involving multiplayer I have discovered and it may even exist beyond 1.15:

 

In Android, skins don't load properly and sometimes your friend's skin renders as yours and yours as theirs and sometimes you both render the same skin but you keep your skin and vice versa ( skin swapping that may occur when two player's have the same skin geometry)

When using addons, custom items don't work well with other players besides the host. It prevents the other players from eating custom items that could always be eaten, foil items from rendering enchantment glint, durability displaying as negative, custom stacked items from being move around the inventory ( sometimes only), stacks don't work correctly (example if the max stack of a custom item is 1, it could stack to 64 to other players and prevent them from moving it around their inventory), animations and hand equipped items don't render correctly.

Sometimes chunks don't load and causes other players, beside the owner of the world to fall through it

Sometimes crashes occur when one person logs off.

And here are some in singleplayer gameplays:

Ender dragon can sometimes hit her own fireballs and knocking it away from you

Area of effect clouds spawned by the enderdragon sometimes are in the ground or doesn't spawn

Sensing component doesn't work when an entity is riding a flying or jumping entity

Berry bushes don't damage you if you don't have armor

Ghast fireballs have a weird offset (try lowering it to -4.1)

And here are some missing parities for 1.15 betas:

Combat update (but gradually and slowly ( maybe start of by adding attack cooldowns and increasing damage of axes and the trident)

Behavior parity of some mobs (like skeletons)

Animation parities

Dual wielding ( also add an order where if one weapon is on cooldown, the left weapon can be used to attack)

Crawling speed and by using trapdoors or pistons

Sneaking under slabs

Tnt explosions dropping all blocks it blew up

Sounds and particle parities

Certain commands (ex: blockdata, sudo, execute if, etc)

Swimming speed

Illusioners ( for addons and map makers)

 

 

 

Apparently I couldn't get any samples for these since I don't have a spare device to test the multiplayer out and my friends are busy, and some of the bugs are rare and couldn't get a glimpse of it but the one with berry bushes what you should try out since it happens to me