So since it hasn't been officially fixed yet AFAIK (mount not found error) I have found a way to fix it in the mean time. I am going to use Wildseed Cradle as the example here, and it will be labeled as WSC. This is assuming you have the latest version of the addon installed, which should be v8.1.5.

1. In game, with the addon enabled, open your mount collection window.
2. While the window is open, find the mount that is not working, like WSC. Keep the window open.
3. Open your chat window, type /id (notice the space after) and then shift click the troubled mount, like WSC.
4. In your chat window you will get an ID number; WSC is 334352.
5. Now that you have the ID number (regardless if it is WSC or not) you need to navigate to your WoW Addon directory, and open the GoGoMountData.lua file (EDIT: this file exists within the GoGoMount folder) with your preferred text editor. NOTE: As always, before modifying a file make a backup copy in case something goes wrong.
6. Scroll down to around line 980 in the file.

What I did is add another commented section (-- shadowlands) under the last section with version number but this is not necessary, just for organization. You can place the following lines wherever you want within the GoGo_Variables.MountDB = {***} array.

Now, this is how you add a mount in the format of the addon. Note that the parts surrounded by <> are what you need to plug in. Do not include either < or >; they are just to distinguish what needs to be changed.

• For a flying mount, this is the format:
  [<ID number from step 4>] = {[9] = true, [38] = true, [300]=true, [301]=true, [330]=true, [400]=true, [402]=true, [403]=true, [405]=true, [701]=true, [10001]=67, [10002]=160, [10003]=250, [10004]=67}, -- <Flying Mount Name>

• For a ground mount, this is the format:
  [<ID number from step 4>] = {[38] = true, [330]=true, [400]=true, [402]=true, [405]=true, [701]=true, [10001]=67, [10002]=160, [10004]=67}, -- <Ground Mount Name>

So just copy and paste the code above for whichever mount you are trying to fix into the GoGoMountData.lua file (wherever in the GoGo_Variables.MountDB = {***} array you choose, and replace the <> with your ID number and Mount Name. After making these changes, save the file then reload your UI and your SL mounts when set as a favorite or global etc. should now work.

Here is what mine currently looks like:

-- shadowlands		
	[332252] = {[9] = true, [38] = true, [300]=true, [301]=true, [330]=true, [400]=true, [402]=true, [403]=true, [405]=true, [701]=true, [10001]=67, [10002]=160, [10003]=250, [10004]=67},  -- Shimmermist Runner
	[334352] = {[9] = true, [38] = true, [300]=true, [301]=true, [330]=true, [400]=true, [402]=true, [403]=true, [405]=true, [701]=true, [10001]=67, [10002]=160, [10003]=250, [10004]=67},  -- Wildseed Cradle
	
	
	[306423] = {[38] = true, [330]=true, [400]=true, [402]=true, [405]=true, [701]=true, [10001]=67, [10002]=160, [10004]=67},  -- Caravan Hyena

If there are other mount types and specific things that require a different template due to their traits. However, this should cover most of the SL mounts.

Hope this helps you guys!

Disclaimer: I am in no way involved with this project. I just wanted to resolve an issue that has not been fixed yet.