See this issue comment: reflexjs/reflexjs#39 (comment).

Tag nodes are created in onCreateNode, which is intended to extend or transform nodes created by other plugins. This implicitly assumes that every node created has a parent node that is properly linked form the newly created node. This assumption is wrong for tag nodes. Tag nodes do not have a single parent node. Therefore, parent is not set and tag nodes get removed from cache.

See gatsbyjs/gatsby#11785 and gatsbyjs/gatsby#12981.

Try sourceNodes lifecycle method.