I now know enough to be sure it's a plymouth bug, not gdm:
https://gitlab.freedesktop.org/plymouth/plymouth/-/issues/118#note_668662
** Changed in: gdm3 (Ubuntu Focal)
Status: Confirmed => Invalid
** Changed in: gdm3 (Ubuntu Groovy)
Status: In Progress => Invalid
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https://bugs.launchpad.net/bugs/1872159
Title:
Boot animation never finishes when external monitors (or just more
than one) are connected
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