The commit introducing the issue is https://github.com/FDio/vpp/commit/2ca88ff97884ec9ed20a853b13cee6d86f9c9d0f#diff-f512dc389f8d0d2a1e0b6fba662f9393cd3c26499e9130e697fddb2b8d514effR102

While before the my_client_index variable was stored as global variable in api_main_t after this patch the my_client_index become also(or should it be only?) part of vapi_ctx_t

So each API client (internal/external) connected to VPP stores it client id in vapi_ctx_t, as we can see here

The issue is the client disconnection. The vapi_disconnect is untouched in this patch, so it keeps the behavior of using the my_client_index stored in api_main_t. As you can check here and here, the message to disconnect from vpp uses am->my_client_index, that was never set when using vapi_connect + vapi_client_connect.