With a recent kernel upgrade, my laptop always wakes from suspend immediately, after having closed and opened the lid at least once. The following illustrates it

1) boot laptop
2.1) send laptop to suspend in any way (except closing the lid)
2.2) laptop remains in suspend as expected
2.3) wake laptop from suspend in any way (except opening the lid)
2.4) repeat from 2.1) ad lib
3.1) close the lid (will suspend if not suspended)
3.2) open the lid (will resume cleanly)

but from now on it looks like this:

4.1) send laptop to suspend in any way (including closing the lid)
4.2) laptop goes to suspend and resumes after a second or two on its own, even with a closed lid.

expected behavior:
the laptop should remain in suspend until the lid is opened or the power button is pressed.

I have confirmed that a downgrade to 4.16.11-gnu-1 removes the issue, so it looks like some recent change has caused this.
There appears to be nothing relevant in the syslog. (no error messages or similar)