I don't see any mention of fertilizer. Did you use any?

I've typically noticed this on cold climate plants that are forced starting in midwinter not march, but I will throw it out there as a possibility. Something caused them to go dormant? Change in light/photoperiod? Exposure to too cold of temps after being in the house at a consistent temperature? When I've seen these winter forced plants go dormant in late spring they do typically resprout sometime in the summer. If the roots look healthy and looks like there are buds where the leaves fell off, wait it out and see what happens. And not all winter forced plants do this, only some species. Others will keep growing through the whole season.

Thank you for the responses!

I did sprinkle a small amount of Osmocote on top of the soil/bark when I potted and used a weak mix of Miracle grow once a week for most of this time. The plants looked so good until a few weeks ago.

I've mixed up 511 several times in the last couple of months so I've lost track but each batch had at least 2 weeks of getting mixed and wet before it was used. The bark looks so chalky on top but my water does leave residue which was why we put a water softener in over a year ago ( I still have a couple sources of the original hard water which I use for my plants).

Dormancy could be it. Been slow to warm up here and have had several days that it didn't get out of the 40's and the plants were outside for a few hours at midday. And there were a couple of periods with week long cold spells (30's to 40's F), that everything stayed in the basement, under 'not so strong' lights (14 hr/day). We've had some low 70's on occasion but mostly 50's and low to mid 60's for highs on most days they've been out, so not ideal growing conditions.

There appears to be beginnings of new buds at the leaf scars.

And the rain barrel filled the other night so I'm able to use rain water again. :-)

I also gave a regular dose of MG to everything yesterday and may just use weak MG mix from now on.

Update:

I finally ditched this leafless plant, actually two of them.

Neither started growing from the buds and I found some extras that came from seeds planted in 'out of the way' places in the yard.

The replacement tree from those seeds, is already close to a foot tall and that's after insects had defoliated the 4" seedling, earlier this season.

I also have a back up in a pot about 12" tall. So these 'dormant' ones are gone, end of story.

New one in ground with 24" tube, healthy large leaves and growing as expected. :-)