RAMDisk disappears after random read tests

I have created 60GB RAMDisk using the command modprobe brd rd_size=62914560. It creates 16 RAMDisks and I use /dev/ram1. The linux is CentOS 7.5 with kernel version 3.10. I don't make any filesystems on RAMDisk because I want to use it as a raw block device. My test scenario includes two phases and I use FIO tool: (1) I write sequentially to /dev/ram1 so that it is initialized and the memory is allocated it. (2) I test the RAMDisk performance with 4KB random read. However, the RAMDisk disappears during the random read test (second phase). I have checked this issue using the command free -m. Why RAMDisk disappears when we read from it?