I forgot to say:
Manually modifying the sub-menu’s inline styles with:
x-placement="bottom-start" and position: absolute; transform: translate3d(0px, 60px, 0px); top: 0px; left: 0px; will-change: transform; does solve it.
I’m just interested to know if there’s a way to interact with PopperJs or Bootstrap’s JS in Sage, rather than hacking stuff.