I defined the braces as MEMBER TENSION but I see these failing in slenderness and the Kl/r limit that these are being checked against is the one corresponding to compression ( 200 ) and not the one corresponding to tension ( 300 ). Can you please explain why ?

First of all please ensure that you are using REPEAT LOADs and NOT LOAD COMBINATIONs for combining the primary load cases when you are using a MEMBER TENSION ( or COMPRESSION ).

If a tension-only member is vertical or sloping, the selfweight of the member will induce a varying axial force which will be zero at the top node and compressive at the bottom node. In combination cases where the selfweight load case is a component, this will cause the axial force to be tensile at the top and compressive at the bottom. In such cases, the nature of the force at the 2 ends cannot be used as a criteria to decide if the member is in tension or in compression. So, the program uses the sign of force at the mid-point of the member. But the fact remains that there will be some compression at the bottom node.

If STAAD finds any hint of compression even at one end of a member, it lowers the slenderness limit to 200. So for these members, although these are specified as MEMBER TENSION, these may be reported as failed in slenderness when Kl/r > the allowable limit for compression. The slenderness check does not practically mean much as the axial force in the member is very small anyway. In such situations you may just bypass the slenderness check for such members by assigning the MAIN parameter and setting it to 1.