That upgrade method is not supported. Do not expect it to work for all use cases.
Perhaps be patient while all upgrade methods are worked on? A lot has to be done in ubuntu-release-upgrader to get certain quirks, such as that, to work.
The bug you linked is old and doesn’t pertain to this.
And as @ashleythorne said, TPM FDE is experimental, and therefore, not supported anyhow.
What you’re asking for may very well be out-of-scope.