A couple of tips.
1: Never release assets from ABM/ASM unless the devices are separating from your organization permanently. Examples of when to release include selling the device, donating the device, distributing the device to employees for personal use, a device damaged beyond repair, a device replaced by Apple under warranty coverage, devices returned at lease expiration, or devices sent to recycling.
2: If you plan to release the device (because of one of the conditions above), you should always release the device BEFORE performing an erase all contents and settings. If you erase first and allow the device to connect to Apple's activation server, the device may cache the activation state at time of first communication. If the asset is still in your ABM and assigned to an MDM, then it could cache the association and cause a nightmare for the person the device is passed to.
3: If you are having re-enrollment issues with Mosyle, I recommend deleting the device record in Mosyle before attempting a reinstall. Unlike Jamf, Mosyle appears to have trouble with re-enrolling devices over existing device records. For my Mosyle deployments, I will always delete the device record from Mosyle before returning a device to service. While Mosyle's inconsistency is maddening, this act generally produces a successful enrollment, allowing Mosyle to create a new device entry instead of associating with an existing record.
Ok, now to your issue. Please note I can only go by what you wrote so I have no magic knowledge of your specific environment. Also, this is a user-to-user forum. Apple has a special call in number for ABM support. Check out this Kbase article for details.
Here is what I recommend.
1: Fully reset the device in question. If this is an ARM based Mac, use an IPSW file fully restore the device. If this is an Intel, boot into Internet Recovery, erase the unit, and then do an Internet restore - yes this takes forever.
2: Using Apple Configurator on your phone, make sure you are signed into your ABM, that you have share WiFi set (unless your WiFi is radius or similar), and preselect your MDM (Mosyle) from Apple Configurator. This will ensure that when adding the device to ABM, it will also add to Mosyle. If you do not do this, the device will add to ABM and be assigned to Apple Configurator as the MDM. This is useless for a DEP enrollment as you are not using Apple Configurator as a tethered MDM.
3: Boot up the wiped device with your phone running Apple Configurator nearby. Capture the device and once added, consider shutting the device down, instead of restarting it immediately.
4: If you are already logged into ABM, log out. ABM has a tendency to show you the state of the system at time of login. So if you logged in 20 minutes ago and then added a device, it may not appear until you expire the session and start over. Log out. Log in fresh.
5: Go to Devices tab and search for the device in question. Confirm that it is assigned to your Mosyle MDM. if not, reassign it.
6: Log into Mosyle. Same as the ABM recommendation above. Since in fresh. Check your DEP token and view the list of assigned devices. Make sure you see the device in the list. If you do not, check the last sync time for the DEP token. If needed, wait a few minutes for Mosyle to sync with ABM. Once you see the device is assigned in Mosyle, then you should be ready to proceed. Ad noted above, if you still have a device record in Mosyle for this device, consider deleting it before attempting to re-enroll.
7: Power up the device and allow for automated enrollment.
Also a side note. I have not side loaded any devices this week so I cannot validate a working process. However, I can point out that about 9 months ago or so, Apple had an issue where side loaded devices were taking up to 24 hours to appear in ABM. If you check the Activity log in ABM you will see that the device was "added." However, it does not appear in the Devices list. Waiting a day and they magically appear. This seems to be a bug that floats around and is never quite "fixed."
If you continue to have trouble, I encourage you to call Apple directly to get support.
Hope this is helpful.
