Hm… Are you perhaps trying to reuse the dev tag? That’s broken in the past, and it’s not completely clear whether Fly.io considers it a supported feature.
(Their examples of --image-label in the official docs suggest that they’re thinking of it as being a fresh tag with each new push.)
The registry is multi-layered, distributed setup these days, so there may be implementation concerns about who still sees the old version of a reused tag, and the like, 
…
(A standard of unique tags, in contrast, creates a monotone system, which is much easier to make distributed.)