Here an excerb of a bug report in the case I'm contacted on GoG.
Other comments highly welcome.

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2.09.4: Keyboard commands don't work for Plasma Launcher + Geo Canon
The keyboard command doesn't work for my secondary weapon in the Multitool which is the Plasma Launcher or the Geo Grenade both with the default key Right Mouse Button.
Remapping to another key doesn't solve the problem.

The bug appears in this particular sequence:
After starting game, going on the planet surface, the keyboard command will work. If I then re-enter my ship and change my location on the planet, I leave my ship, then my keyboard command doesn't work anymore, I cannot fire my grenades until I restart the entire game, after which this sequence will repeat.
Please also test loading screen sequences: passing teleporter, Anomaly, Atlas, warp sequence, black holes. Same issue here after a loading screen.

I'm playing without VR with the GoG offline game version 2.09.4
This bug was already present for me in 2.09.3.

Thank you for a quick fix.
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New info about the bug above:

I've run a test serie to check what I said, and...suddenly all works again, also when changing the key binding. It seems that I'm chasing ghosts :(
All I can say is that the bug will show up for me at some point , but I cannot determine what's causing it atm. (Still in 2.09.4).

I still can't get it to function at all when bound to the middle mouse button "Mouse3." Maybe because that is hard-wired to the terrain manipulator (to switch between dig and build)? I have it bound to "Apostrophe" (back-tick) now which is useable but suboptimal.

I start to believe that this can be caused by ALT-TAB out of the game, I do it often to respond or to post messages in my Firefox Browser. I let sometimes the browser open in background before returning to the game.
I must say that I never have other problems except sometimes falling through the freighter after going-in, but I digress, that's another problem entirely...

Finally the SOLUTION:

Wether you use keyboard or gamepad, this should work:
Quickmenu > Utilities > Change Secondary Weapon

Chose your secondary weapon even if you have it already, this will make the key commands work again. And the controller (?)

I've never seen this command hidden well inside the Quickmenu...Not very practical, but things are like they are :)