The post was deleted. And only users with required privileges can see it.
So, the modification date can be confusing for users without requiring privileges. Could you change the behavior to show the date of the last change that could be viewed?
IMHO we shouldn't do this, because it will lead to even more confusion. If I can't see that modification, and for me the question would be last active 6-8 weeks ago, why does it then show at the top of the list of active questions? One could take this change even further, by showing a different list of active questions for 10k users and sub-10k users, but that already gets more complicated.
Also, this denies sub-10k users the possibility to see that there's something going on with the question. Right now, if I was interested, I could still ask a 10k user in chat if they can tell me what's going on. I remember cases in Charcoal HQ where somebody 'needed' to know this for a possible spam investigation.
An alternative solution to clear the confusion would be to show a popup (e.g. underneath the 'active today' label in the widget on the top right) with a text like
The question was last active today, but you can't view the change because it was on a post that is now deleted.