Ok, after checking better, this seems to be due to the fact that I had to change my .desktop file for insync in order to have KDE display its tray icon under a Wayland session, by prefixing QT_QPA_PLATFORM=xcb to the insync start command (as previously advised by Insync tech support).
Since I have a similar “hack” in place also for Firefox, I have tested autostarting Firefox’s .desktop file, and got the same result, i.e. KDE does not autostart it.
Other apps (dolphin, but also 3rd parties) autostart just fine, so I suspect that autostart of apps is broken in KDE right now, if the command contains some env var override in the .desktop file. I guess I’ll report a bug on the KDE tracker.
The workaround for me meanwhile is to revert the .desktop file for insync to just run “insync start”, at least this way it auto-starts and handles my cloud files - although no tray icon is visible, the process is showing up in System Monitor.
Thank you for sharing more information on the issue! There seems to be an ongoing tray icon issue for users on KDE and I’ve forwarded this issue to our Linux Team.