Did you install from a ventoy USB? I had a weird issue recently where a system was doing something very similar - not a dual boot, just something I was slapping proxmox on. It appeared that a newer version of the ISO was doing something weird with the disk imaging when it tried to copy stuff from ventoy - I saw “ventoy” in some of the paths in the verbose logs on the post-install reboot. Slapping the install image on a bare USB drive and installing from there resolved the issue.
Tangentially: if you’re doing this as an evaluation… from personal experience, I’d just make a full backup of your system disk, then blast it and just make it a dedicated Linux box. Using the dual-boot crutch, in my experience, often devolves into basically just forgetting you even have the Linux partition because you rarely use it, and then a windows update will break grub or something like that, and you just don’t bother fixing it. Doing an actual OS migration is more work, sure, but it also forces you to actually use linux, and solving problems in that context is going to teach you a lot more than bailing out and rebooting into your windows part.
Also also: since you’re already down with KDE, check out kinoite (atomic F40 KDE). Atomic distros are awesome :)