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 ).

There may be situations where you may find small compression to develop at one end of member ( the lower end ) even though the member is specified as MEMBER TENSION. This typically happens when the amount of tension in the member is so small that even the selfweight of the member is enough to nullify the tension and induce a small compression at the lower end while the upper end is still in tension.

The moment 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.