after the last update (2020 January), the "all" button is no longer present during unit selection.
If I want to select all the units, I have to click the buttons "COBRA", "Offence", "Defense", "Recon" one by one (4 clicks) to get everything selected.
the red button you are talking about is not there.
however today the "all" button has finally appeared, as it was before.
I understood which button you were referring to: it is not on the far left (as you said), but on the far right of the series of "red buttons" designed to send the units into the bunker. [see Figure 1].
However, this is not the button I was referring to.
The red button you are talking about has the only task of sending the units to the bunker (or to recall them from the bunker).
I am talking about the unit selection commands in all the other tabs of the game, for example recalling / sending units from / to: MC, Depots, Repos, Base, ZHG Locations, etc. [Figure 2]
After the update, this button was absent until yesterday, February 5th, 2020 [Fig.3], on which day, finally, it mysteriously returned back (luckily!), as this player's comment also testifies:
I hope it is clear now.
in any case, the problem has now been solved by itself.
P.S. (Note)
Next time, please bear in mind that I'm not so stupid that I don't understand that the red button with "soldiers icon" is the new button to send units to the bunker.
So if I say that the "All" button for selecting all units is absent, it means that there is a problem.
