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"^ ^]^^}^
Does MC-249007 describe your issue?