This has been reported before and it is an issue with GTK restoring from the 'minimized to tray' state. There is nothing we can do about this until a fix is applied upstream.
Have you tried without having Deluge maximized or without the 'minimize to tray' option enabled?
Source: https://dev.deluge-torrent.org/ticket/2102
He reported it also upstream, but seemingly ignored and auto-closed 2 years ago when bugtracker changed location and because of long time inactivity.
https://bugzilla.gnome.org/show_bug.cgi?id=675846
That was though gtk2, and this is gtk3, but seemingly still an issue, either since then, or recently brooke again, I dunno. On gtk2 after that report it seemingly got better at a point, so most likely after a gtk2 update, but one reported e.g that it worked fine 5 times in row but then suddenly failed again, so not fully fixed yet there, but just much improved.
Both of these suggested workarounds still work I just tested i.e. the minimize to taskbar works with maximize enabled, and closing to tray works if you manually rezise the unmaximized window once, to fill the full screen out, so as it in effect is "maximized"(well, not technically, and just fullscreen rather, as a distinction from window manager perspective, but regardless) just without selecting the maximized option, and this is retained from tray I just verified.
I'll keep an eye on new gtk3 commits in between releases too, so if a fix comes i'll of-course add it imidiately and not wait for full new release. The same for unmerged pull-requests if applicable. I just checked again now on github as 5 new commits landed since last gtk3 release, but nothing related to that + no issues reported off it on there gitlab bug-tracker, atleast in the 4+ months I just went back through now.
Thanks mate Well, it's unofficial because i'm not a deluge dev. Also it is made in a non standard way because the standard method have issues currently hence the reason for no official installer from real deluge devs. It works the same way though for the end user even though technically differently made than usually. The only difference "from the outside" is little bigger in filesize really.