The bug

Bugged water generates above custom sea-level in swamp biomes.
These bugged water source blocks seen on the screenshots are always found adjacent to other source blocks or terrain, and always on the topmost layer of the swamp.
When a block adjacent to a bugged source block is removed or updated, the water flows in all directions as you would expect.

How to reproduce

1. Create a customized world with the seed being 490287182 and the following preset:

   {"coordinateScale":684.412,"heightScale":888.176,"lowerLimitScale":423.45068,"upperLimitScale":388.2465,"depthNoiseScaleX":409.24646,"depthNoiseScaleZ":409.24646,"depthNoiseScaleExponent":0.8546479,"mainNoiseScaleX":80.0,"mainNoiseScaleY":160.0,"mainNoiseScaleZ":80.0,"baseSize":8.5,"stretchY":12.0,"biomeDepthWeight":0.5,"biomeDepthOffset":0.0,"biomeScaleWeight":2.0704226,"biomeScaleOffset":0.0,"seaLevel":60,"useCaves":true,"useDungeons":true,"dungeonChance":32,"useStrongholds":true,"useVillages":true,"useMineShafts":true,"useTemples":true,"useMonuments":true,"useMansions":true,"useRavines":true,"useWaterLakes":true,"waterLakeChance":4,"useLavaLakes":true,"lavaLakeChance":80,"useLavaOceans":false,"fixedBiome":-1,"biomeSize":4,"riverSize":5,"dirtSize":33,"dirtCount":10,"dirtMinHeight":0,"dirtMaxHeight":256,"gravelSize":33,"gravelCount":8,"gravelMinHeight":0,"gravelMaxHeight":256,"graniteSize":33,"graniteCount":10,"graniteMinHeight":0,"graniteMaxHeight":80,"dioriteSize":33,"dioriteCount":10,"dioriteMinHeight":0,"dioriteMaxHeight":80,"andesiteSize":33,"andesiteCount":10,"andesiteMinHeight":0,"andesiteMaxHeight":80,"coalSize":19,"coalCount":20,"coalMinHeight":0,"coalMaxHeight":128,"ironSize":11,"ironCount":20,"ironMinHeight":0,"ironMaxHeight":64,"goldSize":11,"goldCount":2,"goldMinHeight":0,"goldMaxHeight":32,"redstoneSize":10,"redstoneCount":8,"redstoneMinHeight":0,"redstoneMaxHeight":16,"diamondSize":10,"diamondCount":1,"diamondMinHeight":0,"diamondMaxHeight":16,"lapisSize":9,"lapisCount":1,"lapisCenterHeight":16,"lapisSpread":16}
   

2. /tp 235284 62 70444 80 10