I got into this state by right-clicking on the tab-bar background and choosing "Tab Bar" > "Single Tab". You can't get back to "Multiple Tab" mode the same way because there's no tab-bar background in "Single Tab" mode.

the way back to "Multiple Tab" mode isn't hidden but it might not be the same place you left it from.

this issue was resolved for me with "View" > "Appearance" > "Tab Bar" > "Multiple Tab" (on MacOS)