Where was this memory low alert? If it was from the Z-Wave radio, then likely some stranded devices are misbehaving, which can lead to radio running out of bandwidth.
Based on what you shared thus far, it is not clear how you ended up with only 7 devices? Have you forced remove them? That would explain why they are not listed on Devices page but they are present on Z-Wave Details page. Force removing devices effectively creates ghosts, which could make inclusion/exclusion difficult, if not impossible.
If you didn't check the Z-Wave Troubleshooting document, here is a link to it that may give you some ideas what to look for: How to Troubleshoot Z-Wave | Hubitat Documentation
Last but not least, you can send me a private message and we could look at your hub's engineering logs to see if we can figure out what's going on (assuming your hub is connected to the cloud).
