This doesn't seem to happen with Base UI, but I've checked Shadowed Unit Frames & WeakAuras and they both interact mostly the same.

SUF: If the buff frame the buff occupies hasn't had a timer in it the timer will not show. If the buff frame has had a timer in it, it will show a very incorrect remaining time.

This was with someone who had a 4 day buff.

WeakAuras: This seems to be the opposite of SUF, if its the first time the frame is shown it seems to get the cooldown, although wrong. But if the frame is already created and the duration/exp is added to it or changed, it will stay blank.

This only seems to happen with larger numbers/cooldowns, possible over 1 day? I tried with an hour buff and it worked fine at least.

Could it possibly be linked to changes made with 8.0.6 & #253 ?

For the long duration cooldowns, are you looking at times after rebooting a computer?

The machine had probably been on about 24 Hours or so.