Ok, I checked the sources of the packages: the dolphin (and sub package) sources are equal in F44 and F45, so the difference cannot be contained there. The versions are really equal. This also counts to the package versions we are shown in the settings.
However, in KDE, a lot of stuff is intertwined and what impacts a tool might be not only contained in the tool itself: F45 has many more KDE updates recently compared to F44: 148 to be precise.
However, when I check some of these 148 manually, I can see that in all three cases I checked, they had been already updated on F44 before, so the sources are, again, equal.
So the remaining possibility is that, if one checks all remaining 145 rpms, in any of them F45 has already a newer version, making this difference being not part of dolphin or the packages related to the KDE versions we are shown in the settings. Or, its a customization that is imposed after the source rather than being imposed by the source. The latter can also occur to reference implementations and even be intended by the upstream developers, and our KDE team is heavily intertwined and overlapping with the upstream team, so they usually act in consensus, and they align as far as possible. I could also imagine technical or organizational reasons why that would be not (yet?) set in the upstream source. Or, again, maybe its indeed some change that for whatever reason was introduced by some inconspicuously package/rpm that somehow impacts dolphin.
I still have a tendency to assume the “customization after source” thing, given how the packages and rpms are managed and pushed. But I don’t know KDE architecture sufficiently to make more than a rough guess about this (and I definitely have not the time today to get too deep into this, as curious as I am at the moment
)
So the potential paths of asking are these:
- Why was that implemented
- How (and where) was it implemented
- How to get the old default back to F45
@jandemus must decide which he wants to ask, and in all cases, my first attempt would be to go to the KDE Matrix channel and informally ask there 
This would be not necessarily an override.