When an acacia_log.json file is placed under src/main/resources/data/minecraft/tags/blocks, with the following contents:

{
  "replace": false,
  "values": [
    "minecraft:dirt"
  ]
}

Dirt does not register as an acacia log in recipes or when using clear in-game. Furthermore, if the same file is placed under .../data/modid/tags/blocks, the tag #modid:acacia_logs does not exist in-game.

Probably related to #66

That still appears unsolved, too. And, while perhaps related, I am not confident that they are the same issue.

Probably related to #66

I think this comes from a misunderstanding of how the tags are defined rather than a merely a loading problem. If you add a block tag it won't work in recipes as they use item tags. The same way the /tag command won't work with a block tag as that uses entity tags. As far as I'm aware you have to explicitly re-state a tag if it has one form but you want to use it for another as they're unique per type rather than globally.

Seems to be intentional vanilla behavior since item and block tags are two separate constructs, hence they do not share entries.

Closing.