As I can see Spacemacs is not using the classic yas-expand but uses hippie expand which triggers yas-expand:
M-/ runs the command hippie-expand (found in global-map), which is an
interactive autoloaded compiled Lisp function in ‘hippie-exp.el’.
is bound to M-/.
:around advice: ‘ad-Advice-hippie-expand’
Try to expand text before point, using multiple methods.
The expansion functions in ‘hippie-expand-try-functions-list’ are
tried in order, until a possible expansion is found. Repeated
application of ‘hippie-expand’ inserts successively possible
With a positive numeric argument, jumps directly to the ARG next
function in this list. With a negative argument or just SPC u,
undoes the expansion.