Possibly an oversight?
The end of page fifty is the end of an episode in one edition, and the meeting's resumption is the start of the following week, so it is possible that Hergé simply forgot where he left Snowy in the previous week's adventure.
It could also be that the little scene with Snowy is what in film editing would be called a "jump cut" - moving suddenly from one scene or incident to another, to create a momentary surprise or effect. In this case, the insertion of Snowy's encounter with the technician inserts a moment of humour in a sequence of vignettes which have been building up tension and pressure on the characters.
It doesn't mean that the action is happening at the same time, or even in the same sequence as everything else, just that the mention of it happening triggers the frames in which we see how that testing has been going: the actual event might be earlier in the day, or later, but it illustrates the testing of the suit about which Calculus is talking.
Putting in a little moment of slap-stick comedy at this point in the story would work really well as a release.
The rhythm of the meeting has been to tell us about technical things in a repetitive manner, and is basically admin, which adds authenticity to the tale, but is slightly tedious. So Hergé could use it as a reason to put in a gag or two.
The idea of testing a suit is fine, but custom building a bespoke space-suit for a dog is actually absurd; by making the test we see the least scientifically rigorous thing possible, and then using it to have Snowy hit his nose when tempted by a bone, is light relief among the jargon and mission data.
The preceding pages have made much of the time element, with captions telling us that more and more time is passing throughout the book: "Some days later" (p.47); "The same evening" (p.49); "A few minutes later" (p.50); The same evening..." (p.50); "The days go by..." (p.50) - these all follow to build up the pace.
Hergé overlaps the two strands of narrative (Calculus at the meeting telling us about the testing, and Snowy in the suit being tested) to telescope the narrative, just as a director might on screen, as it intensifies the action.
So to me, not a mistake, just a moment of comic timing and effective time management.Update:
Well, it was good in theory, but looking at the actual dialogue, it doesn't hold much water to be truthful. In English, the sequence of Snowy in his suit is introduced by Calculus as "being finished now
" (my emphasis).
Likewise, in the magazine, the same thing is happening "ce moment"... So regardless of the passing of a week in the real-world between episodes, the testing is happening concurrently with the meeting.
So either there was an event that Hergé chose not to show of the engineer coming to get Snowy, or Snowy went off by himself because he knew he had an appointment in engineering (again without it being shown), or it is simply a little gaffe...! You decide... ;-)