I have a structure like so:
<div class="grid-container fluid">
<div class="grid-x grid-margin-x fluid" id="content">
<div class="cell medium-offset-1 medium-7 small-12>
<div class="content-section">
<div class="media-object stack-for-small">
<div class="media-object-section"> # object1
<div class="thumbnail">
<img src="">
</div>
</div>
<div class="media-object-section"> # object 2
[....]
</div>
</div>
</div>
</div>
<div class="cell medium-3 columns small-12">
Upcoming events...
</div>
</div>
</div>
For large and small screen it works as expected: the stackable items are stacked in small screens and one next to the other for large. The issue is for medium-size displays, where this happens (this is an iPad):
Am I overlooking something obvious?

By default we use the
$-zf-zero-breakpoint(which issmall) and compile the CSS for it there. And this applies it just to the breakpoint small and not medium or others.See https://github.com/foundation/foundation-sites/blob/v6.6.3/scss/components/_media-object.scss#L66 and https://github.com/foundation/foundation-sites/blob/v6.6.3/scss/components/_media-object.scss#L36
If you need it for medium too you can either use the scss version and generate the needed styles using the media-object-stack mixin (https://get.foundation/sites/docs/media-object.html#media-object-stack) or create the needed CSS class manually.
https://github.com/foundation/foundation-sites/blob/v6.6.3/dist/css/foundation.css#L3522
I have done this at https://codepen.io/DanielRuf/pen/LYGRBpq?editors=1100 and added a few spaces and soft hyphens for better automatic text breaks.