Time on lock screen faded

Every so often for the past year or longer, the time on my lock screen would turn grey instead of white, then a few weeks later, it would turn white again, then some weeks later, grey, etc. The sane happens for the widgets just below, but not the text at the bottom.

It happened again today. Restarting the phone fixed it. However, it seems to happen regularly.

Greyed out

Not greyed out

I've read similar threads but haven't seen an explanation for this. For example, it's not low battery mode as that setting dims the whole display but doesn't change the screenshot. Neither does reducing the white point, as this affects everything.

These are my wallpaper settings for the time display:

The one thing I've noticed lately is that it seems to be grey in the second half of the month and white in the first half, but I can't confirm if this was always the case. This month has 31 days and today's the 17th and I first noticed it being grey today. However, restarting the phone made it turn white again.