Excel fail to refresh (repaint) when activating worksheet

Hi,
I have a problem that occured a few weeks ago, and I can't find a way to fix it, hope someone can bring ligth.
In a very VBA intensive workbook, some sheets fail to refresh completely when selected. Part of the sheet, and sometimes the worksheet tabs are greyed out. The worksheet_activate events are triggering a lot of code.

Clicking inside the sheet always causes immediate refresh, and things are ok. For some reason hovering the mouse inside the sheet causes refresh within a few seconds. Doing nothing causes the sheet/application to stay greyed out. The functionality is not affected.

Greyed out means opaque, rectangular grey panels covering parts of the application, it is not the kind of greyed out as appears in inactive menus or the ligth grey fog that covers the application in cases where it becomes unresponsive. It is opaque, grey areas that will disappear with a click of the mouse.

Please see the attachment, the problem will be clearer, I'm unable to explain accurately in plain words.
Wrt the attachement, (1) many sheet tabs missing (including half of the blue "Risk Register" tab, (2) "static" content in sheet header and some column headers greyed out, (3) some columns including half of "E" col greyed out in listobject. The visible content to the right is as it should be.

This behavior is not repeatable, some of the time there are no issues.

And for the sake of it, I don't think it to do with application.enableevens or application.screenupdating etc being applied in the wrong direction.

Using latest version Microsoft365.

Any tips are appreciated:-)

This can happen with extensive event code, where it eats up significant memory.

What is the memory usage of excel when this is running? You can see this in Task Manager.

Right - that makes sense.
Difficult too help further without seeing the code, but there are various code efficiencies that can help (e.g. not 'activating' sheets through code, instead of keeping the current worksheet active and carefully referencing them instead) etc