haha!  I bet nobody ever knew this!  And this is not an April Fool's thing.  Seriously.  It isn't.
Screenshot Captor has the Y2K problem!  It cannot number screenshots past 999!

Really!  So, I set my SC to name screenshots like this:   Screenshot_1.png   Screenshot_2.png
but once it gets to Screenshot_999.png, it can't create Screenshot_1000.png and just keeps overwriting Screenshot_999.png.

Here is the setting at  Settings > File Naming> New Screenshot File Template > Default New File Name
%quickfield% Screenshot%_num%

And, I have checked the box in the same settings section for "Auto sequence new file names to avoid overwriting old files"
Auto-incrementing Number is set to "1"

For myself, I've modified the naming scheme to: %quickfield% Screenshot2022%_num% to get it to start the numbering over at 1.

Maybe it has to do internally with the amount of memory allocated for a possible screenshot to inhabit?  I dunno - just guessing here, seeing how it's likely coded in C++ and the designer must allocate a certain amount of memory before the program can place data into it.  Maybe you've found, via your ultra resolution, the limit.  I bet the program wasn't written to know how to handle that kind of data immensity.