I've faced a same issue on my side, and have also noticed that having Attach Notes and Industrial Display Plates (https://mods.factorio.com/mod/IndustrialDisplays) enabled at the same time also prevents Industrial Display Plates from being blueprinted properly (which works fine if Attach Notes is disabled).
Regarding the sign posts not being included in blueprints, I couldn't reproduce the issue again with clean install and latest release of Factorio/Attach Notes (0.18.35 and 0.4.1, respectively).
As for the Industrial Display Plates, I have posted a wrong link by mistake - I was using the correct (non-deprecated) mod at the time. Sorry about that :)
I have also now retested Attach Notes with Industrial Display Plates, and when both are enabled, copy/pasting (or blueprinting) the industrial display plates will not carry over the item selection on the display plate. But that might be better off in its own discussion if you are interested looking into it (I've already opened thread at Industrial Display Plates mod page regarding the issue)?
azaghal did a lot of code diving (see here) and found that AN is most probably destroying the mapping property of the event on_player_setup_blueprint. I verified this with the infinity cargo wagons from EditorExtensions, which are also missing their filters in blueprints only when AN is enabled.
Since this property is not deprecated, and we have three mods depending on it working and one breaking it, I would see the need to fix on AN's side...
This mod was built way before the mapping and entity tags in blueprints even existed and I had to rely on some pretty hacky workarounds as azaghal has probably noticed. This would need a large rebuild of the mod :/
If I have some time I can look into it, but Im not actively playing this game anymore.