Gravel generated instead of Spawner in Dungeons

What is the world seed and is it a custom generated world?

seed: -5704303796147645466

{"coordinateScale":684.412,"heightScale":684.412,"lowerLimitScale":512.0,"upperLimitScale":512.0,"depthNoiseScaleX":200.0,"depthNoiseScaleZ":200.0,"depthNoiseScaleExponent":0.5,"mainNoiseScaleX":80.0,"mainNoiseScaleY":160.0,"mainNoiseScaleZ":80.0,"baseSize":8.5,"stretchY":12.0,"biomeDepthWeight":1.0,"biomeDepthOffset":0.0,"biomeScaleWeight":1.0,"biomeScaleOffset":0.0,"seaLevel":63,"useCaves":true,"useDungeons":true,"dungeonChance":100,"useStrongholds":true,"useVillages":true,"useMineShafts":true,"useTemples":true,"useRavines":true,"useWaterLakes":true,"waterLakeChance":20,"useLavaLakes":true,"lavaLakeChance":90,"useLavaOceans":false,"fixedBiome":-1,"biomeSize":5,"riverSize":4,"dirtSize":25,"dirtCount":7,"dirtMinHeight":0,"dirtMaxHeight":256,"gravelSize":25,"gravelCount":7,"gravelMinHeight":0,"gravelMaxHeight":256,"graniteSize":25,"graniteCount":7,"graniteMinHeight":0,"graniteMaxHeight":80,"dioriteSize":25,"dioriteCount":7,"dioriteMinHeight":0,"dioriteMaxHeight":80,"andesiteSize":25,"andesiteCount":7,"andesiteMinHeight":0,"andesiteMaxHeight":80,"coalSize":30,"coalCount":17,"coalMinHeight":0,"coalMaxHeight":255,"ironSize":35,"ironCount":19,"ironMinHeight":0,"ironMaxHeight":255,"goldSize":16,"goldCount":10,"goldMinHeight":0,"goldMaxHeight":59,"redstoneSize":20,"redstoneCount":8,"redstoneMinHeight":0,"redstoneMaxHeight":41,"diamondSize":20,"diamondCount":5,"diamondMinHeight":0,"diamondMaxHeight":41,"lapisSize":20,"lapisCount":3,"lapisCenterHeight":21,"lapisSpread":44}

That's strange, re-created world and now there is spawner normally generated with gravel on top...

Also could not reproduce using that preset and seeds, the immediately TP'ing to the coordinates I get to see gravel from the ceiling drop down, but it does not replace any blocks once you dig it out.

I saw it on more places but took screenshot only of this one. Might help you that I was flying in spectator mode when this happened.

Was the world created before 14w20a? Probably related to MC-48442.

Yes, it was. I guess problem solved... Thanks Jono.