The integral sign is the only one which gets mirrored when you switch to RTL — is that how it should behave as it is basically an elongated S (from summation over)?

Or is that a bug in MathML implementation in my browser?

All this library does is put a dir="rtl" on the <math> element. However I think that is per the MathML spec. Only some symbols are supposed to mirror. On both my browsers I see that e.g. the less then and greater then symbols < and > are also mirrored.

That said, there have been some issues with dir="rtl" on Chrome: https://github.com/w3c/mathml-core/issues/288