Sorry to revive this post, but has any progress been made on this issue or can you provide a workaround? For my use case, "primary child tags" being treated as secondary tags makes it impossible for me to use Flarum. I have primary tags (e.g. Cats, Dogs) and sub-tags (e.g. Cat Food under cats and Dog Training under dogs), then I use the secondary tags for information on the state of the discussion (e.g. Needs Help / Pending Solution / Solved), of which it can only have one state. As such, it doesn't make sense for "required number of secondary tags" to apply to the primary sub-tags - because then when someone selects Cat > Cat Food, they can't then select "Open".
I've perused the github issues (and here)and it seems the intention was for primary child tags to be treated as "a greater level of specificity RELATED TO a primary tag" wheras the secondary tags can provide specificity to any other tags, so the current functionality seems to be a bug. Are there any existing extensions or workarounds to address this?