Reflection on adding calmag in reverse osmosis water according to the fertilizers used

hi everyone I would like to have more information on the use of the supplement calmag TA

indeed my tap water having an EC of 0.80 to 0.90 I therefore use a reverse osmosis filter so it made sense to add more calmag but on its product sheet it says “especially not to use with floranova or the special soft water formulas which already contain the sufficient dose even in osmosis water”
and after 2 weeks without calmag my plants showed calcium deficiencies.

So my questions:

how is it possible that floranova is sold as suitable for hard water and soft water knowing that the calcium requirements are different between the 2 types of water?

Has anyone ever had this problem with similar causes?

Thank you in advance for your answers and participation on this subject