最初と最後のポイントを同じように指定する必要があります。
これを試してください。
SET @poly =
'Polygon((-98.07697478272888 30.123832577126326,
-98.07697478272888 30.535734310413392,
-97.48302581787107 30.535734310413392,
-97.48302581787107 30.123832577126326,
-98.07697478272888 30.123832577126326,))';
SELECT name
FROM county_shapes
WHERE MBRContains(ST_GeomFromText(@poly), SHAPE);
AND
SELECT name
FROM county_shapes
WHERE MBRContains(ST_GeomFromText('Polygon((
-98.07697478272888 30.123832577126326,
-98.07697478272888 30.535734310413392,
-97.48302581787107 30.535734310413392,
-97.48302581787107 30.123832577126326,
-98.07697478272888 30.123832577126326))'), SHAPE);