There are two different conditions for what may be the same problem.

Condition A: Space configured as Default Theme renders tabs as text when Global Theme is switched to Classic.

Steps to replicate (make sure during steps 1 and 2 you don't cause condition B below!):

1. Set space theme to Default
2. Set Global theme to Default
3. Change Global theme to Classic
   Browse to a page - the tabs will render incorrectly.

Workaround:
From the error condition, switch the space theme to classic, then back to default.

Condition B: Space configured as Classic Theme renders additional tabs when Global theme is switched to Default
It looks like the inverse of Condition A:

1. Set Space to Default
2. Set Global to Classic (again, make sure you don't cause condition A when you set this part up)
3. Switch Global theme to Default

Workaround: Switch the space theme to Classic and then back to default.