Timing change - not really....although tulips grown in containers tend to do badly as perennials, splitting off into daughter (non-flowering) bulbs after one season. Many people grow them just as annuals....but most, not all, can be perennialised by planting deep in well drained soil which is dryish in winter and gets a good summer baking. They are fairly late flowering, coming into bloom at the same time as the lily-flowered bulbs (around end of April, first couple of weeks in May) and should start to extend daily from now.

The weather is the significant factor. Blooming time here has varied by 3 weeks (used to keep yearly calendar to try to co-ordinate everything - Hah!) Plants pay attention to Mother Nature. 'Menton' may be slow to develop its buds?

Read an article years ago by a noted tulip hybridizer. He stated all tulips are splitters. Doesn't matter how deep they are planted nor how cold the winter. The splits do eventually grow to blooming size. I have seen this over the past 10 years with a group of red Darwins.

My favorite tulip, 'Apricot Beauty' belongs to the Triumph group and he said this group is the exception and does not perrenialize for the home gardener. They die out.
The Holland bulb growers do not reveal their secret of getting triumphs to blooming size.

I have many tulips which have been returning for well over a decade.....tulips of all classes, including Triumphs. True, some lose the massive overblown blooms - to my mind a good thing since the smaller compact plants are better equipped to deal with my open, sunny plot in windy East Anglia. It is not the winter cold which is an issue - it is having a summer baking in hot dry soil (where the bulbs can go into a deep dormancy) which seems to do the trick - as mine are usually along the edges of every veggie bed, they are never watered or moved from one year to the next.....although I have, on ocassion, grown lettuce over the top of them.....older gardeners have memories of digging and storing tulips every year, in order to accomplish the same thing.....and is a similar method used by commercial bulb-growers, to separate the daughter bulbs, grow them on for one or 2 years while not letting them flower, then digging the bulbs for sale.

It sounds like you're doing a good job with your potted tulips. They can handle a little overnight freeze ( lows in the high 20's), so if you get tired of lugging them in every night there is some leeway....
They'll take a little longer to develop their flowers as compared to the other tulips. Right when the Darwins are ending they'll open. I love them! If I think of it later I'll look for a picture from last year.... The year I dug them up and killed them all by storing them damp :(