I think I have finally found the solution. After a discussion with Crucial support, they suggested changing the BIOS from RAID to AHCI mode. I was able to do that following instructions from here ([URL] thankfully didn't need to enter my BitLocker key but I backed it up anyway. I've now gone well over a week without the drive disappearing, with many resumes from sleep.
I fitted a second SSD in my XPS 15 9510 (I added a Crucial P2 M.2 2280 1TB SSD) a few weeks ago. It works just fine, but I started noticing that very often when I turned the PC on after a few hours away (so presumably it has been in sleep mode), the 2nd SSD (my D drive) is missing. Rebooting always brings it back.  I've tried reseating the drive twice, and I'm sure that it is firmly placed in the slot. And I installed the "Crucial Storage Executive" which reports that the SSD is healthy. I saw some other people with similar issues of SSDs disappearing after wake solving it by disabling "hybrid sleep" but that's not an option on my power settings.  I'm fully up to date with all drivers and firmware, so I'm at a loss to know what else to try. Any suggestions?
XPS 9510 2nd SSD often disappears after wake