This issue was first opened 2014. It was promised to be fixed years ago, yet it is still an issue.
In some cases it is impossible to have a brush stroke look correctly on closed paths and suggested workarounds are painful or not sufficient.
Whether this is a bug or we need a way to tweak the gap/start/end point etc I don't know, I just need a solution that is manageable.
Could we please have the developers take a look at this again?
https://forum.affinity.serif.com/index.php