Fence posts, as well as walls, glass panes, iron bars and torches connect to a number of non-solid blocks listed below. This probably results from the improved connectivity towards stairs but can't really be intentional.
Affects:
sticky pistons:
|| ||front||back||side||
||retracted| yes | yes | yes |
||extended| yes | yes | no |normal pistons:
|| ||front||back||side||
||retracted| no | no | no |
||extended| yes | no | no |
No longer affects:
(Fixed in 17w16b)
nether portals
(Fixed in 17w16a)
banners
buttons
dragon eggs
fire
levers
pressure plates
rails
redstone wires
signs
structure voids
sugar cane
tripwire
tripwire hooks
vines
Linked issues
is duplicated by 33
Attachments
Comments 12

The fact that signs and banners connect, kind of make sense? No more flying infront of fences!
But you are right, when signs are standing on the ground they should not attach. Otherwise they should

Can confirm (17w15a)
It's already marked as confirmed, no need to comment that anymore.
Oh god, attached sings can't connect either. I can't count how many furniture constructions that would break in my world.

Okay, it seems fences didn't connect to dragon eggs in previous versions, so I think they are affected here.
Don't forget torches (rs and light) on this list.

Fixed for everything except nether portal blocks in 17w16a.

Also still affects pistons and sticky pistons in 17w16a and 17w16b.
Changed expected results from pistons: if it's retracted all sides are full, and as such it's logical that it connects, for extended the sides are the only ones expected not to attach, so to keep it clean the retracted pistons should not connect with their sides.
Can confirm