Browsers don't restore tabs after quitting in macOS Mojave
Looks like you have found a bug,
I can reproduce your issue on my Mac running 10.14.1

It does work after logging out (and back in) or force closing Safari,
but when I close it normally and reopen, it does not reopen my last session,
until I click reopen last closed tab, and it suddenly restores all my tabs from the last session. Bit annoying but at least a work around (Cmd + Shift + T)

I needs a bit more testing I think, but looks like this should be reported to Apple if this behaviour is indeed consistent.