Why do the frame ticks disapear when a plot is exported as PNG?

I am using M12.0.0.0 on Ubuntu and I have these two plots:

plot1 = DensityPlot[Sin[x y], {x, -2, 2}, {y, 0, 4}, PlotPoints -> 80]; plot2 = Plot[Sin[x], {x, -10, 10}, Frame -> True]; 

when I export these plots as png the FramTicks disapear! here is plot1 as seen in the MMA notebook

enter image description here
and this how it looks when exported using Export["/.../plotq1.png", plot1,ImageResolution -> 500]
enter image description here

same thing also applies for plot2 in the notebook is like

enter image description here

and exported is like

enter image description here