Tags not kept?

Dropbox/Google drive might strip the metadata.

If the external drive is formatted ExFAT or FAT32, then the metadata would get stripped. Those formats cannot store metadata which is where the tags are stored.

Tags on files copied to another Mac, where those custom tags do not exist on the target Mac behave as follows where source Mac is Mojave 10.14.6 (18G5033), and destination Mac is Catalina 10.15.5 (19F101 - supplemental):

1. AirDrop
   1. Custom tag name is automatically entered and activated in Finder Preferences : Tags with same color, and tag appearing on destination filename.
2. Copy -p src dst through Terminal
   1. Copying a file in Terminal from a src Mac to dst Mac via iCloud Drive automatically enters the custom tag name in the destination Mac's Finder Preferences : Tags panel, but it is neither activated, nor is a color assigned.
3. Finder copy to iCloud Drive
   1. Same result as [2]. When viewed with Finder on iCloud Drive from 2nd Mac, no tag will appear assigned, though it is still on the file. One will have to visit Finder Preferences : Tags to activate, and apply color to the tag on that file.
4. Finder copy to FAT32 formatted USB stick
   1. The destination Mac Finder will show the document icon with custom tag color while located on the FAT32 filesystem, and the custom tag entry will also appear in Finder Preferences : Tags with the color set, but not activated.

I don't know if this will help you troubleshoot, but you can view the metadata stored before and after the move by using the mdls command in Terminal. Type mdls and space, then drag a tagged file into the Terminal window. Hit return.

All of the metadata will be displayed. The tags are in this item: kMDItemUserTags

mdls is the "Metadata List" command and is all lowercase (MDLS).

Once transferred back to an Apple formatted drive, it will create the basic metadata for the file to be indexed by spotlight, but any special metadata like Tags cannot be created if they don't already exist.

How did you move them?

If you moved them via an external drive that does not have a mac-compatible file system, the metadata may have been stripped from the files.