Why should any normal user have to check to see if the problem still occurs in the latest developmental software?
Why should someone have to install two different versions of the software just to see if they can find one that works?
Imagine someone has a two-year-old car that they find a fault with. They take it to the garage and the technician says that it's a known inherent fault that's been fixed in the prototype of the next version. The manufacturer won't be retrofitting a fix to older cars but the customer can borrow the prototype if they want to. However, the manufacturer doesn't consider the prototype to be road-legal so it might crash because of some other unknown fault. Oh, and the manufacturer doesn't know when the next road-legal version of the car will be ready. None of that helps the customer who has to drive the car they already own.
If the user wants to use the current stable version of Scribus - because that's the version that the Scribus team itself considers to be the most stable version for production purposes - then anything that's been changed in the developmental code is irrelevant to that user.
If there's a bug in the stable version - which is the version that the majority of people are using - then there's a bug in the version that most people are using. In other words, most people have a version of the software that contains the bug.
If the software was being developed in a way where new stable versions came out fairly regularly, there wouldn't be so much of an issue. All the user would have to do was wait a few months and everything would be fine. If 1.5.0 was a stable version when it came out then that would have been fine. The same with 1.5.1, 1.5.2, 1.5.3, and 1.5.4 when that comes out. Having to wait more than two years for a fix to the version that most people are using puts those users in a very confusing and infuriating position. Do they use the stable version with its known issues, or use the developmental version with its unknown issues, or use some weird mixture of both?
If the problem being reported has been fixed in 1.5.x then that's great for people who are using that version, but it's irrelevant to the people who are using the version that is recommended for use by the team itself. Especially if there is no way for people to know when the next stable version might be available.
Personally I have no plans to move to 1.5.x. I can do what I want with 1.4.6 and if I have more complex needs then I have other software that I can use (either instead of, or in conjunction with, Scribus). Not everyone is in the same situation however. A lot of people can only use Scribus and they are being put in the position where they have to choose between using software that hasn't been updated since 2016 or software that has any number of unknown problems.
I really don't have much more energy to put into this discussion. The same issues keep coming round again and again, but they can be stopped once 1.6 is released. Once there is a new stable version - even if it does contain lots of bugs - everyone can be using the same version of the software and everyone can upgrade when the next set of bugs have been fixed. Surely that's a better way of doing things?