Unfortunately I have messed with this a great number of times and the issue is that Blender and roblox define bones differently. Exporting from blender to Roblox works because Roblox knows how to convert the bones, whereas vice versa does not, because Roblox does not use bones or export a rig with bones and blender does not know how to treat them, so essentially when the rig is exported and imported into Blender, the bones “disappear”
If you put bones back, you must have the exact correct amount used in Roblox rigs and must rename each bone of the character to the exact correct “bone name” for it to properly export out to roblox again once you recreate the rig. (For this, I found the documentation to be incomplete so I downloaded and imported a rig package from Roblox’s own provided rig files and looked at what each mesh piece was named and named my bones accordingly.)
When I am on PC tomorrow I can provide some resource links, but overall it is honestly easier to just rig it yourself quickly rather than trying to wrangle it out of Roblox. Exporting between the two doesn’t always play nice, as seen in this case.