I've not tried your preset because is too late now.
But I'm sorry, when you assign an ext amp to a FS, the on/off state of that FS is snapshot dependent.
Any change of snapshot will recall the state stored in that snapshot. This behavior makes the assignment of the ext amp command to a FS basically useless (unless you need it to work that way).
Works exactly how helix worked before fw 3.5 when the use of both direct parameter assignments to footswitches and snapshots was impossible.
That's why they added a snapshot control option for all parameters.
(And they added the snapshot bypass options because having a blocks that can stay
on or off regardless which snapshot you're in is incredibly useful)
I'll add, as a side note, that using a FS for simply changing a tube amp channel makes little sense, you can bring the actual amp pedal and free a footswitch on your helix.
All that said, is nonetheless cool having all the switching on your helix floor with all the color coding and scribble strips.
anyway, what makes really sense is integrate the amp channel switching capabilities with other stuff inside the helix, like turning on/off things, adding a compressor when you select a clean channel, or selecting different routes in the signal path or modify your reverb mix and length, etc.