Most brine tanks are now equipped with a safety float and valve, to stop water during a malfunction or power outage during Brine Fill, from continuing to overfill the brine tank once the liquid level has risen to a specific height within the tank. Due to this, many softener brine tank's now supplied, no longer include an overflow drain, but some suppliers continue to include one since an overflow drain had been a standard inclusion previously.Also my brine tank was never connected with a overflow drain so it would just pour out on the floor is there was a problem. I think it has to have its own drain separate from the softener drain?
If you are nervous, there is no harm if you wish to include a drain tube from the brine tank fitting to a floor drain or similar separate lower gravity drain not directly connected to the softener's drain line. You are correct that connecting the brine tank overflow fitting directly to the pressurized drain line from the softener, may cause the brine tank to incorrectly fill with water that is intended to flow to drain during regeneration.