When connecting up onto a non-full block, redstone will not actually visually connect on the side of the block when going up said block, even though it does actually connect when powered. This has been the case for a long while but as 20w18a and onward plan to fix redstone visual logic this should probably be paid attention to.
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Affects 1.16 pre5
Can confirm in 20w51a.
Can confirm in 21w05b.
Can confirm in 21w06a.
Can confirm in 1.16.5 and 21w08b.
Can confirm in 21w10a. Video attached.
Can confirm in 21w15a.
Can confirm in 1.17.
Can confirm in 1.17.1.

If RS wire were to connect completely upwards, this is how it would look like (for visualization purposes I additionally used a pack that alters the wire model):
[media][media]
In my personal opinion, the wire upwards would look more logical if there was no wire in the "air gap", but solely where an actual block was (at slabs' side):
[media]
If memory serves well, I achieved that back in 2014 or so by adding an additional block model - the visualization pack I used for half of the screenshots uses 2 additional wire block models as well, to circumvent the optical gap of upwards-redstone other model packs have.
This is a tough one, the three options I can think of are:
Make redstone not connect to upper slabs. Pro: Makes sense logically/to beginners. Con: Breaks many devices dependent on this behavior.
Visually connect the redstone over the bottom half. Pro: Shows connection to player/beginner. Con: Floating redstone dust on bottom half of slab.
Leave it the way it is now. Pro: Nothing breaks&No Floating Redstone. Con: Doesn't clearly show connection to player.