Yes Braiins software takes 2.5% of your hashrate by periodically switching to what is called a DevFee pool and yes when it does you are not mining elsewhere. Stratum has an optional feature which can be enabled called Nsub that lets a miner seamlessly switch work to a different pool/coin without rebooting and you having no indication when it does that. The DevFee config is hard coded into the Braiins (or other 3rd party) software. It is also often considered a security hole because you have NO idea what it is doing and when. In this case you are knowingly allowing it to switch to the DevFee pool and you have to trust Braiins for that to be all it does.

While you can use a tool like Wireshark to find the IP of the pool and then block it, doing so will result in the miner shutting down until it is again allowed to 'phone home' so Braiins (or Vnish et al) get paid for you using their software.