I posted just before you, check the bottom of page 1.
My salt changes don't correlate with the times where my tank went from my care to the tank sitter's care. The first time I changed to H2Ocean was while the tank sitter was looking after the tank, but he had been looking after the tank for many months before the salt change. This last salt change was several weeks before the tank sitter was looking after the tank. I tend not to make a bunch of changes at a time so I can see how my tank reacts to each change so I know which ones are worthwhile. I had similar questions as you, and didn't really believe it was the salt until I made the change this time and looked back in my documentation and photos, and now I'm fairly sure it is indeed the salt. Fwiw, the tank sitter is actually doing quite a good job on the 90-gallon reef this time. There was some algae bloom, so I changed my feeding protocol for him and upgraded from two Koralia 3 to a Tunze 6105 4 weeks ago and added a second 6105 on Christmas Day. This upgrade created a much more significant change in the tank than the salt change ever did.
I think a lot of people try a new salt for one bucket which for some people may only be a couple months of waterchanges. I don't think that is enough. I think you need at least 6 months to assess a different salt. I took about a year.