Garmin removed all older versions from their public servers many years ago.

Note there's a gap in Garmin's Change History from V6.20 to V.6.6:
Spoiler:
Changes made from version 6.70 to 6.80:
  • Improved the display quality in junction view.

Changes made from version 6.60 to 6.70:
  • Improved software stability.

Changes made from version 6.20 to 6.60:
  • Added support for a new digital traffic provider in Germany.

The inference to be drawn is that there were at least 3 fw versions that were released only to be later withdrawn, V6.30 to V6.50. Garmin always deletes badly failed fw releases from its change log. If a proposed version fails internal testing it's not released generally and that's also the case if limited user testing is unsatisfactory, i.e. with "UAT" versions. So the next release following, say V6.20, is V6.30 as we would expect and we'd likely never even know of the failed Alpha, Beta or UAT version at all. Clearly on at least 3 occasions they've less loose faulty RTM versions in the recent past. Additionally, if you look at the earlier change logs there's been more such occasions because there are many other gaps:
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My point is that going back to an earlier version will probably fix your current RLT errors but potentially might introduce other problems. My best guess is that there is some basic problem with the DA firmware that's eluded their efforts to correct, and the various band-aid solutions have not solved that or have fixed one problem and started another. As it's been a year since V6.80 they may well have now simply given up on it. Anyway, here's V6.20 for you to try if you want: [Only registered and activated users can see links. ]

I also have 3.40 and 3.50 and you might even be able to find others on the defunct Perry site via the Wayback Machine, for example:
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