Does that happening only in pinball or while playing other games too?
And what keyboard you have, or are you using laptop? Some laptops/keyboard have that problem (from what Im reading atm), that you cannot press two buttons at once.

I've only noticed it in pinball, no other games (then again, I don't spend a lot of time in games so call this comment inconclusive)
The keyboard itself appears to be a fairly standard 101/102 keyboard - HP branded, on a slim desktop profile computer.

So - workaround... use two non related keys.

I have the Pinball Games working with Left-Shift / Right-Ctrl. Non ideal but it works.

I have found the same problem (with the same fix) now in Timeshock pinball also.

Strangely enough, Fantastic Journey Pinball works fine with left-shirt / right-shift. This makes me think its not keyboard driver related - but I am open to suggestions!

Very late reply- new to GOG and the game (so good to have my Amiga fave back after so long!) but still relevant should anyone else get similar issue hence worth replying If you ever have stiff flipper problems (ahem) and your flippers ain't flipping as you expect- exit the game and see if there's a clicking sound as you type- if so (or even if not so just in case):

I had this happen today but the solution came when I realized after exiting the game that my keys had started making an annoying low click sound as I typed and sometimes weren't typing characters or there was a delay- looked this up and I'd somehow turned on FILTER KEYS for the keyboard settings in Windows 10. Hence the clicking and delays- and broken flippers in Dreams.

So it could be this rather than a game bug.

To check if it's your issue go into the Windows control panel, ease of access keyboard settings sections and find 'make the keyboard easier to use'- if filter keys has a tick uncheck the box to turn it off, this had my flippers working fine and as they were without problems, think filter keys ignores keys clicked in quick succession sometimes or something like that- changes how PC responds to keyboard input. Probably that paperclip trying to be helpful again!