The bug appears when you have Windows regional settings: dot as grouping symbol and comma as decimal sepearator. When you try to set value to Decimal field in MS ACCESS using AdoConnection + AdoTable, you get actual value multiplied with precision, ie: you set 12,354 you get 12354 for Decimal(18,3) field in MDB.
table['Filed'] := 12.354
FireDac MSAcc driver doesn't meet this bug. The bug appears on both providers Microsoft.Jet.OLEDB.4.0 Microsoft.ACE.OLEDB.12.0
Bug tracking: https://quality.embarcadero.com/browse/RSP-34075