As far as I'm aware, Minecraft 1.12 added different-coloured beds, but had to store their colour in a block entity or such to avoid running out of block IDs. That's not relevant as of Minecraft 1.13, though, and the different beds are all different blocks now for sure, which means that they could be rendered as different block models just fine, but they are still rendered as entities (which can't even be changed through resource packs). I believe that that's a "bug" (or at least a problem that a fix for is not just a "suggestion"), as when they're rendered as entities, they have several issues that they wouldn't if they were rendered as block models (like most other blocks).

(TL;DR: Minecraft 1.12 rendered new coloured beds as entities to save block IDs, but beds are still entities even though Minecraft 1.13 made block IDs plentiful, and this is a problem.)

Issues that are a result of beds being rendered as entities instead of blocks:

• Beds disappear from a certain distance (64 blocks/4 chunks?). (This is probably the most notable problem here.)
• Beds can't be re-modelled through resource packs like most other blocks can.
• Beds can't have their sides "cull" against other blocks (meaning that the bottom of beds' feet are rendered even if the bed is on top of a full block, and that all sides are rendered even if the bed is buried).
• The "breaking" animation when "mining" the bed in Survival mode is not centred on the middle of the beds' sides. (Not super important, but still an annoying thing in my opinion. This is also avoidable when rendered as an entity, though, but still, it's an issue that affects beds at the moment.)

Again, you could argue that for some of those things, this report is more of a suggestion to make them render as blocks than a bug report about them not doing so... but beds being invisible from a distance is definitely a bug, and beds not "culling" against other blocks is also definitely not an intended behaviour, so I classify this as a bug.