Howdy Baribal,
Thank you for your bug report. I am currently exploring the issue that you are having and just have a quick question to help with the setup. When you say that have hollow BSP brushes, I assume that there are maybe 4 or more hollow BSP boxes connecting to one another and they have a subtractive brush to create doors, correct? Also, If you have any simple reproduction steps or additional information that you could provide, it would be greatly appreciated.
Thanks and have a great day!