It can be a firmware bug or Webupdater bug, it happened in the past too.
The point here is that Garmin is providing these updates via WebUpdater/GE to GTM units that were not meant to accept the update because of a different Product ID. This is the main cause of confusion. WebUpdater/GE reports one thing while actually checking the GTM's firmware reveals that nothing has to be updated in some GTM units.
But check if you really have the latest update for your traffic receiver:
and read all the info of your receiver.
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Check using the diagnostics with the unit connected to the GTM:
From the Map screen:
1. Press the Speed field
2. Press the compass heading for some seconds.
3. Press Developer Info
4. Scroll down to and press Traffic Receiver Status
GTM35 Part number is: 006-B0901-00 verified update to firmware v3.10
If you got a successful update you should have 006B090100.GCD in \.System\RemoteSW