I’m trying to group by year then month with the same date field but it seems like Metabase is only able to group results once. Is this a limitation or am I looking at it incorrectly?
Not a limitation, you're just doing it wrong 
Grouping by month gives month-year by default.
Limitation is if you want a stacked bar chart with one stack for each year divided by month. For that, you'll need to alias the column by converting the question to SQL.
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