This is a native Linux export, so as far as I know, Steam will only run that with Steam Linux Runtime 1.0.
The depot is set up correctly because the correct binaries are winding up where they should be. This was all working perfectly up until I re-built the binaries with encryption. The re-built binaries DO work correctly on my Ubuntu 26.04 desktop development system.
On the Steam Deck, I switched to desktop mode and ran the encrypted binary from the command line:
./<appname>.x86_64: /usr/lib/libm.so.6: version `GLIBC_2.43’ not found (required by ./<appname>.x86_64)
Interestingly, while searching this specific message together with Steam Deck, I arrived right back on this forum 20 days ago, where you had plenty of helpful advice!
There is a solution in that thread, but it may be more trouble than it’s worth, given that this problem is only arising with encrypted builds, which I certainly don’t have to use. It raises another question, though: why are only the encrypted builds failing? I built export templates against the same Godot version that my regular unencrypted exports were using.