It is actually the 5e rules. The offhand attack (Bonus Action) does not add Str nor Dex, unless you have the Fighter specific feature or other similar features that bypass this "penalty".
ERRATUM: Wait, nevermind : you mean you are playing Fighters with the dual-wielding style. Roger.
In my combat logs, I think I saw the bonus damage on offhand even for non-Fighters (which would be an error). Will have to check some more.
ERRATUM 2 : yoops, the topic is Attack Rolls :P ... Will check next time. Cheers
Last edited by Baraz; 12/10/20 04:49 PM.