bug#25309: 11.89.8; multi-level script fontification stacks incorrectly

[jfbu]

I get the script raising to work fine, but under the condition
of typing explicitely braces, i.e. 

\( x^{y^{z^a_b}} \)

does the expected thing from the docs after having set to
multi-level the ‘font-latex-fontify-script’, but the thing
about electric scripts does not seem to work

\( x^y_z \) does not insert the braces while typing
even with 

% Local variables:
% TeX-electric-sub-and-superscript: t
% End:

and reloading the file

Best,

Jean-François