with newest release (1.35.0-wmf.19), it seems that gallery slideshow stopped working when not wrapped in a block element with "max-width", "min-width". or "width" style defined.
before this version, it was enough to have a:
<gallery mode=slideshow> file | description .... </gallery>
for gallery to actually show a slideshow. from this version, it seems that it became essential to either add style with set max-width to the gallery tag, or to wrap it inside a div with max-width value, like so:
<gallery mode="slideshow" style="max-width:438px"> file | description .... </gallery>
or like so
<div style="max-width:438px"> <gallery mode=slideshow> file | description .... </gallery> </div>
otherwise, no slideshow.
this is not documented anywhere, and i find it hard to believe it's the intended behavior.
verified with chrome ( 79.0.3945.130), firefox (73.0.1) and edge (44.18362.449.0).