DISCLAIMER: might not be a "bug" per se, but the documentation should warn about this following behaviour:
I had the same issue as this guy:
#854810300876062773 message
It only occurs when Stack align is set to center flex-start or flex-end
As a consequence, the Divider width is shrunk to 0 and is invisible
I fixed it by adding a width style equal to 100%.
I guess it's just something to be aware of.