Hi,
I think there is a bug on S5321-28P-SI-AC switches;
This switch has four SFP ports on it and that looks like 1G ports.
The switch has the phrase 1G below that ports.
But in real life when you try to make a stack connection between two of this model switch with using 1G modules it gives you an error, when you put 10G spfs (Single Mode) on this ports and try to make a stack connection, it works fine.
I have tested it with a HUWAEI brand 1G Single Mode SPFs and FINISAR brand 10G Single Mode SPFs and it works fine when you put 10Gs instead of 1Gs.
This should be a bug :)