Calling ViewBox.autoRange() multiple times lead to different results if the plotted data points have all identical y-values. For example, when you plot an array like [1.0, 1.0, ... 1.0], as I do in the mwe below.

The reason for that can be found in ViewBox.setRange(rect, padding, ...), where for this special case of 0 extent of the vertical axis, the current visible range shall be preserved, see also comment "If we requested 0 range, try to preserve previous scale." in line 594.
But with padding being not equal to 0.0, the current visible range is not preserved, but rather enlarged by the padding.

The only way I see to circumvent this is to add some other special handling here.

Here a mwe which simply plots some data as described above und further calls ViewBox.autoRange() when you click into the widget.

import pyqtgraph as pg
from PySide2 import QtCore


app = pg.mkQApp()

class ClickablePlotWidget(pg.PlotWidget):
    clicked: QtCore.Signal = QtCore.Signal()

    def mousePressEvent(self, ev):
        self.clicked.emit()
        super().mousePressEvent(ev)


p = ClickablePlotWidget()
p.show()

p.plot([1.0, 1.0, 1.0])  # bad
# p.plot([1.0, 10.0, 1.0])  # good

def _autorange():
    print("ViewBox.autoRange()")
    vb = p.plotItem.getViewBox()
    vb.autoRange()
    # vb.autoRange(padding=0.0)  # workaround

p.clicked.connect(_autorange)


if __name__ == '__main__':
    pg.exec()