I hear you. We spend all this money thinking apex is the best but its like a toy that sometimes works. Apex needs to maybe spend a little money and make there products better and higher quilty materials.
They key to the apex salinity probe is placement, if it gets any air bubbles in the tip, or is in a area with micro bubbles, it won't be accurate.
I watched your video where you were adding salt and talking about it also. And I noticed in my own tank when my salt was low and so was my big 3 that it wasn't accurate at all. Once I corrected it, it has been spot on for close to a year. I Make my salt at 1.025 on my refractometer. And my Apex always says 33.3-33.4 ppt. which is the equivalent to 1.025-1.0252. The apex probe is actually a conductivity probe that the apex uses a algorithm to convert that to ppt of salinity. I believe part of that algorithm assumes the parameters are within normal range. Where as in your video your salt was in the 20's. Just my 2 cents