Thanks for the report, knott!
This is however expected behaviour for Trail, and can happen in many similar scenarios where you can discard multiple Trails.
> The logic of the client is that the Trails are not really played (until played 1-by1 sequentially)
> but part of the deck until played (they can be drawn again in the new deck), but that seems weird to me.
The Trails are all discarded, so they are all in your discard pile. That's what Tactician does.
Then you play the first Trail, it moves from your discard pile into play and gives you +1 Card, +1 Action, so you draw a card.
Then you play a second Trail, it moves from your discard pile into play and gives you +1 Card, +1 Action, so you draw another card.
This goes happily on until finally you play a Trail when your deck is empty. It goes into play, then before you can draw a card you have to shuffle your discard pile, then draw a card. If you have discarded Trails that you didn't play yet, they have been shuffled into your deck.
And at this point a rule kicks in that exists since around 15 month, you can read Donald X.'s announcement here: http://forum.dominionstrategy.com/index.php?topic=19893.msg871558#msg871558
The rule is: An effect that tries to play a card for the first time can only do so when the card is where the effect expects it to be. If it can play it the first time, subsequent plays will also work.
The wording about "first time" is not relevant here, Trail only plays itself once. But when a yet unplayed discarded Trail has been shuffled into the deck, it is no longer "where the effect expects it to be" (which is the discard pile where it was discarded to).
And the reason for this rule should be apparent now: You'd have to look through your deck to find those Trails, which is something that should not happen.
So in a nutshell, since the card representing the effect cannot "be found", the card cannot be played.