ARobin I'm using the latest version of Solus -- are you?
There is no "version" of Solus, I assume you're asking if my system is up to date. I'm running unstable and update multiple times a day. In fact, I just updated, so yes.
ARobin There is only one way to install flatpaks on Solus: flatpak install.
This is incorrect. If you install Discover (the KDE Software Center), you can install flatpaks with it.
ARobin The thing is, following your directions results in the flatpaks not loading at all, even when you click on the run icon in the Autostart menu
Flatpaks not loading at all is related, but different. I have quite a few flatpaks I installed through the cli, some of which I use daily. The only flatpak that refuses to launch for me is LibreWolf, which is known to be fraught. If you can reliably reproduce this, please open a bug report with one or two examples of the flatpaks that don't launch at all, the exact flatpak full name, and how exactly you installed it so we can determine if it's something in Solus or something in the flatpak. We do want these to work reliably.
If the Autostart entry pointed to an install that's deleted, it would still have an entry but fail to launch from the start icon. For a particular flatpak, can you let us know which one, and what the details were if you click on the entry in the Autostart list? That will have an error message with more details.
For me, after installing KClock, as I said, it ran. It also auto-started on reboot / re-log after I added it to Autostart programs. I didn't set "Allow executing file as program" or do anything other than add it to Autostart. My prior test was on Plasma built from git-master, so to make sure it behaves the same in 6.3.5, I re-tested with my non-development session. It works there too.
ARobin For example, Forecast will always run, but sometimes it won't connect to it's server in the Internet.
That sounds like an issue with either their server or the network between your machine and the server. That's something to report to whoever distributes the Forecast flatpak. (There are two apps in flatpak search named "Forecast" so I didn't test it).
kClock will always run, but it is almost always invisible (every once in awhile it will work). I have to delete the the Clock process and manually run kClock before it will run.
I did run into this after a couple of re-logs (both in git-master and Plasma 6.3.5). That also sound like an issue with that flatpak. The .desktop test will narrow it down.