Since IE11 does not support Element.prototype.closest, it falls back to the included polyfill. This polyfill fails if element happens to be an SVG since SVGs do not have the parentElement property in Internet Explorer. The polyfill then tries to access undefined.parentElement.

A possible fix would be to use parentNode instead of parentElement or check whether element is undefined in the do...while loop. I don't think that this would break anything, but I don't know enough about the inner workings of bootstrap-vue.