I also ran into this problem in QGIS 3 and found this solution in stack overflow
Basically the idea would be to apply the angle on the polygon on which the extent is defined before creating the grid. If your polygon is not a rectangle, you would need to create a layer from the extent of your polygon prior, and then rotate it. Then you can create the grid according to this new extent and then rotate your polygon and the grid back to the original Polygon extent. All this while making sure the same x, y coordinate is used as anchor point in both layers.
#Define extent of Polygon
ext = QgsVectorLayer('path_to_polygon.shp', '', 'ogr' ).extent()
xmin = ext.xMinimum()
xmax = ext.xMaximum()
ymin = ext.yMinimum()
ymax = ext.yMaximum()
coords = "%f,%f,%f,%f" %(xmin, xmax, ymin, ymax)
#Define The angle of rotation. Change value to yours
azimut = 70.043
#define anchor point for rotation
anchor = "%f, %f" % (xmin, ymax)
#define x and y spacing of grid. Update to your desired spacing.
x = 3
y = 6
#create new polygon from extent
processing.run("native:extenttolayer", {'INPUT':coords,'OUTPUT':'Path_to_Output.shp'})
#Rotate Extent
processing.run("native:rotatefeatures", {'INPUT': 'Path_to_extent_Polygon.shp','ANGLE': azimut,'ANCHOR':anchor + '[EPSG:4326]','OUTPUT': 'Path_to_rotated_extent.shp'})
#Define extent of Rotated Polygon
ext1 = QgsVectorLayer('Path_to_Rotated_Extent.shp', '', 'ogr' ).extent()
xmin1 = ext1.xMinimum()
xmax1 = ext1.xMaximum()
ymin1 = ext1.yMinimum()
ymax1 = ext1.yMaximum()
coords1 = "%f,%f,%f,%f" %(xmin1, xmax1, ymin1, ymax1)
#Create grid
processing.run("qgis:creategrid", {'TYPE':0,'EXTENT': coords1 +'[EPSG:4326]','HSPACING':x,'VSPACING':y,'HOVERLAY':0,'VOVERLAY':0,'CRS':'EPSG:4326','OUTPUT': 'Path_to_grid.shp'})
#Rotate Grid to original extent
processing.run("native:rotatefeatures", {'INPUT': 'path_to_grid.shp','ANGLE': -
azimut,'ANCHOR':rotate + '[EPSG:4326]','OUTPUT': 'path_to_rotated_grid.shp'})
# Clip Grid to Original Polygon
processing.run("native:clip", {'INPUT':'path_to_rotated_grid.shp','OVERLAY':
'path_to_original_Polygon.shp','OUTPUT':'path_to_final_grid.shp'})