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Might be the vaccum check batteries indeed, but you should be getting a clear warning that the battery needs changing soon, when switching on the vaccum system: instead of blinking blue as normal, it would alternate between blue and red.
If it eventually settles on blue, that means you can proceed with vaccuming and should be fine for the next dive or so. If it settles on red, that means the battery needs changing now (it won’t let you do the vaccuming procedure).
I’d be surprised if air was sucked through the HDMI interface, but even if it was, it should settle at some point.
Your vaccum valve/system might be faulty, one way to rule this out would be to uninstall the monitor, blank the M24 port as before and see if you still encounter issues.
If this doesn’t help, it would be worth asking your housing retailer for guidance, keep us posted!