Unfortunately, the issue persists after doing the latest updates including an Octave update.

In T6686#124820, @sunnyflunk wrote:

The theme is basically no theme (who knows what the default is), cause clearing that variable unsets making it look like your GTK theme (which breaks octave as you've experienced). May have to see what can be done in terms of a fallback theme...GTK desktops don't tend to be able to change the settings of Qt themeing.

Can also try QT_QPA_PLATFORMTHEME="gtk3" octave --gui but may be equally bad

I can confirm the issue on two different machines running Budgie (Intel HD Graphics 520) and Mate (nVidia Ion). To reproduce the issue, the file browser window must be opened (Ctrl + Shift + 2) - this is the case using the default window layout though.

How can I downgrade until it's fixed?

As far as I know, Solus doesn't support display scaling on X11 - so it might be a sane default to set this enviromental variable QT_AUTO_SCREEN_SCALE_FACTOR=0 to deactivate Qt Autoscaling as described in this Qt Blog entry globally.
Issue also mentioned here and here.

I can confirm the mentioned issue.

In T730#39068, @der_eismann wrote:

@Justin The package works fine for me, but it seems like it is lacking Shibboleth authentication which is crucial for use with my university. Would you kindly try to build it with Shibboleth support?
(See here: https://forum.seafile.com/t/how-can-i-enable-shibboleth-sign-on-using-seafile-client-under-fedora/1170/3)

I had the same problem the other day installing Solus in a Dual-Boot configuration alongside Windows 10. Deleting "ESP/EFI/Boot/bootx64.efi" on the EFI partition solved the problem and reinstalling Solus was successful even though my EFI Partition which was created by Windows 10 was only 99MB - not 512MB as suggested by https://wiki.solus-project.com/UEFI
99MB seems to be the standard size of Windows 10's EFI partition