Repeating task every 6 months does not sync properly with Google Calendar

I am using the Google Two-Way Sync plugin with Quire. When I create a task in Quire and set it to repeat every 6 months (6 can be any number of months), I notice that the event is created in Google, but the repeat defaults to every 1 month.

After some testing, I found that daily and weekly frequency do sync correctly, but both monthly and annual exhibit the behavior described above - i.e. they default to frequency of 1 month or 1 year.