diff --git a/subsys/net/lib/lwm2m/lwm2m_engine.c b/subsys/net/lib/lwm2m/lwm2m_engine.c
index 194b45c92ff..c5b1ddfb780 100644
--- a/subsys/net/lib/lwm2m/lwm2m_engine.c
+++ b/subsys/net/lib/lwm2m/lwm2m_engine.c
@@ -1361,6 +1361,7 @@ int lwm2m_engine_set_res_data(char *pathstr, void *data_ptr, uint16_t data_len,
 	/* assign data elements */
 	res_inst->data_ptr = data_ptr;
 	res_inst->data_len = data_len;
+	res_inst->max_data_len = data_len;
 	res_inst->data_flags = data_flags;
 
 	return ret;
@@ -1374,7 +1375,7 @@ static int lwm2m_engine_set(char *pathstr, void *value, uint16_t len)
 	struct lwm2m_engine_res *res = NULL;
 	struct lwm2m_engine_res_inst *res_inst = NULL;
 	void *data_ptr = NULL;
-	size_t data_len = 0;
+	size_t max_data_len = 0;
 	int ret = 0;
 	bool changed = false;
 
@@ -1411,13 +1412,13 @@ static int lwm2m_engine_set(char *pathstr, void *value, uint16_t len)
 
 	/* setup initial data elements */
 	data_ptr = res_inst->data_ptr;
-	data_len = res_inst->data_len;
+	max_data_len = res_inst->max_data_len;
 
 	/* allow user to override data elements via callback */
 	if (res->pre_write_cb) {
 		data_ptr = res->pre_write_cb(obj_inst->obj_inst_id,
 					     res->res_id, res_inst->res_inst_id,
-					     &data_len);
+					     &max_data_len);
 	}
 
 	if (!data_ptr) {
@@ -1428,7 +1429,7 @@ static int lwm2m_engine_set(char *pathstr, void *value, uint16_t len)
 	}
 
 	/* check length (note: we add 1 to string length for NULL pad) */
-	if (len > res_inst->data_len -
+	if (len > res_inst->max_data_len -
 		(obj_field->data_type == LWM2M_RES_TYPE_STRING ? 1 : 0)) {
 		LOG_ERR("length %u is too long for res instance %d data",
 			len, path.res_id);
@@ -1512,6 +1513,8 @@ static int lwm2m_engine_set(char *pathstr, void *value, uint16_t len)
 
 	}
 
+	res_inst->data_len = len;
+
 	if (res->post_write_cb) {
 		ret = res->post_write_cb(obj_inst->obj_inst_id,
 					 res->res_id, res_inst->res_inst_id,
@@ -1951,6 +1954,7 @@ int lwm2m_engine_delete_res_inst(char *pathstr)
 	}
 
 	res_inst->data_ptr = NULL;
+	res_inst->max_data_len = 0U;
 	res_inst->data_len = 0U;
 	res_inst->res_inst_id = RES_INSTANCE_NOT_CREATED;
 
@@ -2273,7 +2277,7 @@ static int lwm2m_write_handler_opaque(struct lwm2m_engine_obj_inst *obj_inst,
 		in->block_ctx->opaque = opaque_ctx;
 	}
 
-	return ret;
+	return opaque_ctx.len;
 }
 
 /* This function is exposed for the content format writers */
@@ -2302,7 +2306,7 @@ int lwm2m_write_handler(struct lwm2m_engine_obj_inst *obj_inst,
 
 	/* setup initial data elements */
 	data_ptr = res_inst->data_ptr;
-	data_len = res_inst->data_len;
+	data_len = res_inst->max_data_len;
 
 	/* allow user to override data elements via callback */
 	if (res->pre_write_cb) {
@@ -2334,6 +2338,7 @@ int lwm2m_write_handler(struct lwm2m_engine_obj_inst *obj_inst,
 				return ret;
 			}
 
+			len = ret;
 			break;
 
 		case LWM2M_RES_TYPE_STRING:
@@ -2421,6 +2426,8 @@ int lwm2m_write_handler(struct lwm2m_engine_obj_inst *obj_inst,
 		return -ENOENT;
 	}
 
+	res_inst->data_len = len;
+
 	if (res->post_write_cb &&
 	    obj_field->data_type != LWM2M_RES_TYPE_OPAQUE) {
 		ret = res->post_write_cb(obj_inst->obj_inst_id,
diff --git a/subsys/net/lib/lwm2m/lwm2m_obj_firmware_pull.c b/subsys/net/lib/lwm2m/lwm2m_obj_firmware_pull.c
index 156ca4dee22..dae27012255 100644
--- a/subsys/net/lib/lwm2m/lwm2m_obj_firmware_pull.c
+++ b/subsys/net/lib/lwm2m/lwm2m_obj_firmware_pull.c
@@ -314,7 +314,7 @@ do_firmware_transfer_reply_cb(const struct coap_packet *response,
 
 		/* get buffer data */
 		write_buf = res->res_instances->data_ptr;
-		write_buflen = res->res_instances->data_len;
+		write_buflen = res->res_instances->max_data_len;
 
 		/* check for user override to buffer */
 		if (res->pre_write_cb) {
diff --git a/subsys/net/lib/lwm2m/lwm2m_object.h b/subsys/net/lib/lwm2m/lwm2m_object.h
index ef5955ab965..63206fe0026 100644
--- a/subsys/net/lib/lwm2m/lwm2m_object.h
+++ b/subsys/net/lib/lwm2m/lwm2m_object.h
@@ -214,6 +214,8 @@ struct lwm2m_engine_obj {
 			for (int _i = 0; _i < _ri_count; _i++) { \
 				_ri_ptr[_ri_idx + _i].data_ptr = \
 						(_data_ptr + _i); \
+				_ri_ptr[_ri_idx + _i].max_data_len = \
+						_data_len; \
 				_ri_ptr[_ri_idx + _i].data_len = \
 						_data_len; \
 				if (_ri_create) { \
@@ -233,6 +235,7 @@ struct lwm2m_engine_obj {
 		if (_ri_count > 0) { \
 			for (int _i = 0; _i < _ri_count; _i++) { \
 				_ri_ptr[_ri_idx + _i].data_ptr = NULL; \
+				_ri_ptr[_ri_idx + _i].max_data_len = 0; \
 				_ri_ptr[_ri_idx + _i].data_len = 0; \
 				if (_ri_create) { \
 					_ri_ptr[_ri_idx + _i].res_inst_id = \
@@ -323,6 +326,7 @@ struct lwm2m_attr {
 
 struct lwm2m_engine_res_inst {
 	void  *data_ptr;
+	uint16_t max_data_len;
 	uint16_t data_len;
 	uint16_t res_inst_id; /* 65535 == not "created" */
 	uint8_t  data_flags;