Due to the MC-902 fix, in cases where the Obsidian platform generates inside the island, the End Stone all drops in order to replace it with air. So the first time a player enters the End, it results in 75 End Stone laying on the floor.
Exemple of seed where this happens: 115001344559158446
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The amount of blocks dropped does not matter. Sometimes the obsidian platform can generate with some of it enclosed in blocks and some of it not.
It's actually 100 end stone, since 100 blocks total are replaced (75 with air and 25 with obsidian), assuming the platform generated fully underground.
As for the fix, maybe make an end stone a special case, i.e. it gets deleted without a trace instead of dropped. Or, if you're feeling more ambitious, keep the current behavior, but simply move the spawn platform to a location farther out and higher up, with a pre-generated bridge providing easy access to the island. This would also allow players to go into the dragon fight without having to worry as much about ender pearls and building blocks.