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By default, the
Preferences > General: Open project(s) on startup behavior is set to
Last active project.
And a default project template file (used for new projects) can optionally be set under
General > Project.
So, as Starpainter already mentioned: if one of them (last active project or default project template) is loaded at launch and includes any (vst) plugins, these plugins need to be scanned/loaded.
Or it might also be related to a combination of the things mentioned in this thread (post #6 and #8):
https://forum.cockos.com/showthread.php?t=207787