In July 2014 I got the PS4-version of "TowerFall: Ascencion" into my Instant Game Collection. In December 2015 the Vita-version was release and it was supposed to be a Cross-Buy title.

I never checked if my Vita-version was correctly activated in 2015 or 2016, but there was apparently a problem in the EU Store: https://www.reddit.com/r/vita/comments/4p927m/towerfall_ascension_euau_not_cross_buy/

When I checked the PSN Easter Sale today, I saw the Vita-version of "TowerFall: Ascencion" for free (probably because of the active PS4-version). After logging out, the price changed to €4.99.

So I bought it for free and downloaded it to my Vita. I expected the normal expiry date of my other PS+ games, but when I checked the information, there was no expiry date. I also checked my PS4-version and now that expiry date is gone, too.

Are there other PS+ games without expiry date? Did this happen for TowerFall Ascension because they forgot Cross-Buy at first and added it later?

I've never heard of that! It sounds like a glitch to me, they gave it to you for free but the transaction was as if you had paid for it.

It's quite common to happen with cross-buy titles. Jak & Daxter trilogy is also owned permanently not tied to your ps+ subscription.