It is not so bizarre if coupled to two things: One, Apple planned to release it a bit later, but the 'old' 4K monitor supply ran out unexpectedly (e.g. they turned of production when they thought they had a 4-month supply and only had 2.5 months). The spotty store coverage would line up with this, too...Here's more on this unannounced Apple-LG mystery monitor update:
Julio Ojeda-Zapata said:Apple Debuts LG’s All-New 23.7-inch UltraFine Display
Did Apple just stealth-release a new Mac display?
I’m not talking about the much-rumored 6K display that the company is supposedly designing and building for use with the upcoming new version of the Mac Pro. We might hear about such a product at Apple’s Worldwide Developers Conference, which kicks off 3 June 2019 (see “The Top Conferences for Mac and iOS Professionals in 2019,” 19 May 2019).
No, I’m referring to a third-party monitor, the apparently new 23.7-inch LG UltraFine Display, that, until I went to an Apple Store last week, I knew nothing about. And it seems that few others have heard of it....
Two, it is a better bridge between a shift where all the Mac laptops go to Thunderbolt 3 and the current set up, where they have the MacBook outlier on Type-C DisplayPort alternate mode. The 'old' 4K monitor was only geared to cover the latter (and the USB extension sockets were limited to just USB 2.0 bandwidth). According to the manual, this [new] display can either be fed via Thunderbolt v3 or DisplayPort alt mode (but the resolutions may not be the same). So Apple swapped in a less expensive panel for adding Thunderbolt v3 and kept the price the same.
I think the 9to5 article (linkded in the Tidbits one) notes that this should also work with iPad Pro. Similarly, if Apple flipped a "laptop" to ARM, and it had same external display limitations as iPad Pro, this would work. This may "fit" better into the Apple ecosystem after the WWDC events finish outlining where iOS on iPad Pro is going.
P.S. I don't think Apple upgraded the non-Retina iMac (the "edu" / "entry") model. Perhaps they may be using this "slightly less than Retina" TFT panel in a refresh of that to finally dump the 1080p TFT panel. Eventually, this could be used as the panel in an edu/entry price-sensitive iMac upgrade. It is a more mainstream panel, so they get "volume" out of the greater market. (The 21.5" is basically 'custom' to Apple at this point.) The Retinas could move to a similarly sized screen (to share the same basic enclosure), but 'better' (color, backlighting, resolution, refresh, and/or bezel size).