I can't modify (merge) a player's NBT data directly using the data (or old entitydata) command, but I can still read it.
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is duplicated by 5
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Explanation on why this is intended: https://www.reddit.com/r/MinecraftCommands/comments/9ty42s/data_modify_is_irritating_me_today/e91rwc7/?context=3
This is a lie! I have friends who are engaged in the development of modifications, and they say that minecraft has a vanila packet, with which you can update the player's data on the server. Mojang is deliberately forbidden to do this for some unknown reason, this once again proves that bad programmers work in mojang.
100% intended. MC-120693 for instance