please try this patch to your optee.
diff --git a/core/arch/arm/kernel/stmm_sp.c b/core/arch/arm/kernel/stmm_sp.c
index 1d6344d..6441c03 100644
--- a/core/arch/arm/kernel/stmm_sp.c
+++ b/core/arch/arm/kernel/stmm_sp.c
@@ -77,7 +77,7 @@
static const uint16_t ffa_storage_id = 4U;
static const unsigned int stmm_stack_size = 4 * SMALL_PAGE_SIZE;
-static const unsigned int stmm_heap_size = 750 * SMALL_PAGE_SIZE;
+static const unsigned int stmm_heap_size = 1024 * SMALL_PAGE_SIZE;
static const unsigned int stmm_sec_buf_size = 21 * SMALL_PAGE_SIZE;
static const unsigned int stmm_ns_comm_buf_size = 21 * SMALL_PAGE_SIZE;