Found expression: UTMMajorLabel(round(Y_min(map_get( item_variables( 'main'),'map_extent')),0))
../src/core/expression/qgsexpression.cpp: 441: (replaceExpressionText) [0ms] Expression parser error: syntax error, unexpected '(', expecting $end

import sys
from qgis.core import (
    QgsProject, QgsComposition, QgsApplication, QgsProviderRegistry, QgsLayoutManager, QgsMapLayerStore, QgsAtlasComposition, QgsComposerMap)
from qgis.gui import QgsMapCanvas, QgsLayerTreeMapCanvasBridge

##Set these
#export QGIS_PREFIX_PATH=/usr/local/Cellar/qgis3-dev/2.99/QGIS.app/Contents/MacOS
#export PYTHONPATH=/usr/local/Cellar/qgis3-dev/2.99/lib/python3.6/site-packages/


gui_flag = False
QgsApplication.setPrefixPath("/usr/local/Cellar/qgis3-dev/2.99/QGIS.app/Contents/MacOS", True)
app = QgsApplication([], gui_flag)
app.initQgis()

project_path = 'blahblahblah.qgs'

canvas = QgsMapCanvas()
project = QgsProject.instance()
project.read(project_path)
bridge = QgsLayerTreeMapCanvasBridge(
  QgsProject.instance().layerTreeRoot(), canvas)
bridge.setCanvasLayers()

layoutManager=project.layoutManager()
comps=layoutManager.compositions()
composition=layoutManager.compositionByName('Print1')

atlasMap = composition.getComposerMapById(7)
print("Atlas Composer Map: {}".format(atlasMap.displayName()))

atlas = QgsAtlasComposition(composition)
coverage = project.layerStore().mapLayersByName("coverage")[0]
atlas.setCoverageLayer(coverage)

atlas.setEnabled(True)
atlas.setHideCoverage(False)

atlasMap.setFrameEnabled( True);
composition.setAtlasMode(2)

atlasMap.setAtlasDriven(True)
atlasMap.setAtlasScalingMode( QgsComposerMap.Auto )

atlas.beginRender()
print('{} Atlas Features'.format(atlas.numFeatures()))

for i in range(0, 3):
     print('page: {} {}'.format(i,atlas.nameForPage(i)))
     atlas.prepareForFeature( i )
     atlas.
     image = composition.printPageAsRaster(0)
     fname=('{}.jpg'.format(i))
     image.save(fname)
atlas.endRender()

QgsProject.instance().clear()
QgsApplication.exitQgis()