Even with "Grab all keys" option checked, dead keys don't work properly. They are simply ignored, other than with a following space bar, which generates only the corresponding diacritic: ['] + [ ] generates ['], ['] + [e] generates [e], and so on.
Is there some other configuration parameter that may affect the keyboard behavior I should consider?
Remmina 1.4.5 on Uqbuntu 20.04; connection with Windows 8.1
Possibly related question: https://unix.stackexchange.com/questions/57725/remmina-doesnt-eat-keys
It seems that the 'Use the client keyboard layout' general option (RDP section) also matters; when checked, dead keys don't work properly (as described), even if both keyboard layouts are the same (US International).
Unchecking it resolves the problem.