Operator Precedence problem?

I have a peculiar problem that I can't reproduce on a trivial database:

select product, priority, count(*) as completed from issue
where
product = 'DIS'
and create_date < '1-Aug-2000' and finish_date >= '1-Aug-2000' or
finish_date is null
group by product, priority;

This produces a list of all products - not just 'DIS'. If I put the last
two clauses in parnthesis, then it works as expected:

select product, priority, count(*) as completed from issue
where
product = 'DIS'
and create_date < '1-Aug-2000' and (finish_date >= '1-Aug-2000' or
finish_date is null)
group by product, priority;

Which makes me think that the precedence of 'or' is not what I expected. Is
this a feature? If so, the fact that I get precisely the opposite behaviour
in simple test databases must be a bug, I think.

Any help or explanation would be appreciated...

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