from decimal import Decimal

print(-0.9//0.123)
# prints -8.0
print(Decimal('-0.9')//Decimal('0.123'))
# prints -7
print(-10//4.2)
# prints -3.0
print(Decimal('-10')//Decimal('4.2'))
# prints -2

The behaviour is deliberate, if unfortunate: it's covered in the documentation here: https://docs.python.org/3/library/decimal.html#decimal-objects - see the paragraph starting

> There are some small differences between arithmetic on Decimal objects
> and arithmetic on integers and floats. When the remainder operator % is
> applied to Decimal objects

The issue is that the decimal spec specifies "divide-integer" and "remainder" operations. We've chosen to map those operations to "%" and "//" for convenience, even though there's a difference between float and Decimal here.

Thanks for the report. I'm going to close here, since this isn't a bug.

If you want to advocate for a behaviour change, by all means go ahead, but be aware that it would likely be a hard sell. The main challenge would be finding a way to change the behaviour that doesn't abruptly break existing code that depends on the current semantics.