Not right at all!
If you have been buying 30 gallons+ a week from that same LFS that lies to you, that sure could be your problem. Who knows what "oops" ah well... quality of water you got in the past.
Surely they sell plain RO water for top off, check the TDS. They could easily have two RODI, one for salt mix and one for regular but you could ask to test their all RO's! Assuming you won't be going back of course. Are they a sponsor?
I wouldn't go to a store that I know lied to me. As a matter of fact there is a LFS that I won't ever step foot into because of just that!