bootstrap-flex "mobile first"/xs columns aren't working unless explicitly declared

[mejobloggs]

This code works great on normal bootstrap 4 (and 3)

<style type="text/css">
    .col-sm-4 { border: 1px solid black; }
</style>

<div class="row">
    <div class="col-sm-4">aaa aaa aaa</div>
    <div class="col-sm-4">bbb bbb bbb bbb bbb bbb</div>
    <div class="col-sm-4">ccc ccc ccc ccc ccc ccc ccc ccc ccc</div>
</div>

As expected, you have 3 equal columns until you get down to XS, then you have 3, 100% vertically stacked columns like this:
XS screen without Flex

However, when you enable Flex support, SM and up is still fine, but XS is not. This is what you get:

XS screen with Flex

Or... if there's enough room to fit more on a line, you will get this:

To get the desired column behavior with Flex I have to explicitly declare the XS columns like so:

<div class="row">
    <div class="col-xs-12 col-sm-4">aaa aaa aaa</div>
    <div class="col-xs-12 col-sm-4">bbb bbb bbb bbb bbb bbb</div>
    <div class="col-xs-12 col-sm-4">ccc ccc ccc ccc ccc ccc ccc ccc ccc</div>
</div>

Which in XS screen gives us:

Can we get it fixed so we don't have to explicitly declare col-xs-12?

[cvrebert]

Duplicate of #17603. Please search more in the future. Thanks!