Interesting issue! Can you please post your model? much easier to do some testing directly in your model
What version of SW are you using!
Having issues with mirrored features has been a problem in various conditions, including parts and assemblies. My recommendation is using Solid Bodies to Mirror instead of features. If that didn't work, then don't mirror but instead create that feature yourself.
If this issue is reproducible on a different file as well, send it to your VAR so that they can report it to SOLIDWORKS as service request.
Hope this helps.