I unfortunately have no idea how to replicate this, all I can say that might be useful is that I tend to create a lot of new worlds because I don't use the /reload function whenever I change something in my datapack, I instead create a new world to test things.

But now I was messing around and I found out that despite having no datapacks enabled at all, block and entity tags from a custom datapack were present.

The following video perfectly demonstrates what is happening, watch for yourself on the F3 screen at the side for the block tags: (I wasn't showcasing entity tags in the video but I tried it off-camera and they also work the same way)

Link to YT video: (sorry, I don't know how to embed here)

https://www.youtube.com/watch?v=Z9fySJ8XUOU

If it helps, here is the #time:air_valid json:
^{{
    "values":[
    "minecraft:air",
    "minecraft:void_air",
    "minecraft:cave_air",
    "minecraft:water",
    "minecraft:lava",
    "minecraft:grass",
    "#minecraft:flowers",
    "minecraft:fern",
    "minecraft:fire",
    "minecraft:tall_grass",
    "minecraft:large_fern",
    "minecraft:vine",
    "minecraft:twisting_vines",
    "minecraft:twisting_vines_plant",
    "minecraft:weeping_vines",
    "minecraft:weeping_vines_plant",
    "#minecraft:crops",
    "#minecraft:saplings",
    "#minecraft:signs",
    "#minecraft:buttons",
    "minecraft:attached_melon_stem",
    "minecraft:attached_pumpkin_stem",
    "minecraft:nether_wart",
    "minecraft:sweet_berry_bush",
    "minecraft:cocoa",
    "minecraft:sugar_cane",
    "minecraft:dead_bush",
    "minecraft:seagrass",
    "minecraft:tall_seagrass",
    "minecraft:redstone_wire",
    "minecraft:rail",
    "minecraft:powered_rail",
    "minecraft:activator_rail",
    "minecraft:detector_rail",
    "minecraft:torch",
    "minecraft:soul_torch",
    "minecraft:redstone_torch",
    "minecraft:glow_lichen",
    "minecraft:snow",
    "minecraft:tripwire",
    "minecraft:mangrove_propagule",
    "minecraft:frogspawn",
    "minecraft:sculk_vein"
    ]
}}