Empty boot seqence after using live USB

Hi,

I have some problems with my Dell Latitude E7470. A while ago I installed a debian on it and everything runs smoothly, except for one part. When I boot from a live USB device with any other linux distribution on it, the boot sequence in the bios gets erased.

At first I thought there is a problem with the partitions and reinstalled. The system worked again smoothly until I used a live USB the next time. After that I went into the bios and saw that the boot sequence was empty. I manually restored the sequence and got my system to boot without reinstalling. 

My question is, how to prevent the boot sequence from being erased? I would like to use specialized live systems once in a while without having to restore everything manually again. 

I also updated the bios to the latest version, but that didn't fix the issue.

Many thanks!