Spawners give inconsistent spawn results in 1.14. With strict spawn delays(ALL set to 1, though I have set them all to 0 in a few tests and that also does not work properly) and a timer to shut down the spawner after a set amount of ticks, the spawner sometimes gives a varied number of spawns. This has never happened before 1.14, and persists throughout all of the 1.14 releases. In versions 1.10(have not tested it in any versions prior to 1.10) all the way through 1.13.2, the spawner gives exactly the intended number of spawns every time.

I have tested it with both a spawner block and a minecart_spawner, results were both the same(varied spawns). I have also tested it with both a command block timer and a regular redstone timer - results were the same(varied spawns).

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How to reproduce:

1. See attached picture, copy the small setup

2. First command block(on the left): 

summon spawner_minecart ~ ~1 ~ {id: spawner_minecart,OnGround: 0,MaxNearbyEntities:100,Type: 0,Items: [],Delay: 1,MinSpawnDelay: 1,MaxSpawndelay: 1,SpawnCount: 1,SpawnRange:0,RequiredPlayerRange: 32767,SpawnData:{id:"item",Item:{id:stone,Count:1}},SpawnPotentials: [{ Entity:{id:"item",Item:{id:dirt,Damage:0,Count:1,tag:{display:{Lore:["\"Creeper\""]}}}},Weight:30},{ Entity:{id:"item",Item:{id:dirt,Damage:0,Count:1,tag:{display:{Lore:["\"Skeleton\""]}}}},Weight:30},{ Entity:{id:"item",Item:{id:dirt,Damage:0,Count:1,tag:{display:{Lore:[None]}}}},Weight:0}]}

3. Second command block(on the right):

kill @e[type=minecraft:spawner_minecart,distance=..5]

4. Activate the repeater at the start

In versions 1.13.2 and before, this will always result in a minecart spawner spawning 2 items before being deleted. In versions 1.14 and beyond, it will sometimes spawn 1 item, and sometimes spawn 2 items. No pattern known.