I like to keep things organized in my blueprints, which means I like to use reroute nodes to keep TODO comments.
But, I figured out soon that comments on the reroute node is hiding when mouse is hovering over the node after compile blueprint.
I think it’s a bug because the rest of the node comments are all fine.
It’s pretty annoying, agreed. Made me stop using re-route comments. And it’s been there since ever, I think. A tiny QoL thing unlikely to end up as a high priority fix. You can still try, though:
They misspelled “has been a known issue for 3+ years.” 
Thanks for the update!
I fixed it myself.
If someone interested in fixing this issue and possible to build UE4 engine, only need to delete just three lines.
Delete every following lines in SGraphNodeKnot.cpp
CommentBubble->SetCommentBubbleVisibility(/*bVisible =*/ ... );