umm, i do have bookmarks toolbar always show on my setups but that shouldn't make a difference (edit: tested with it set to only show on new tabs, doesn't make a difference)
both nightly and alpha exhibit this behavior (OP was about nightly)
I'm still unable to reproduce on Linux, on MacOS (where rather than "maximize" we've got a hybrid "fullscreen") and on a Windows VM, but I finally (and consistently) could on my wife's Windows 11 notebook.
Therefore I've kidnapped said notebook to try understand what at this moment seems a Windows-only (and maybe timing-related, re: VM non-reproducibility) bug.
ma1changed title from when remember maximized, restore is fubared to When startup window is maximized per letterboxing.rememberSize on startup, the restore button shrinks it to its minimum size
changed title from when remember maximized, restore is fubared to When startup window is maximized per letterboxing.rememberSize on startup, the restore button shrinks it to its minimum size