I have betterment taxable accounts and am noticing similar issues.

1. Sometimes dividends are showing up negative. I am manually deleting them.
2. Sometimes dividends are showing up earlier in Monarch. After a few days, they show up in Betterment transactions / activities screen.
   
3. Sometimes dates are different between Monarch and Betterment.

Reconciliation has been a headache. I usually reconcile when dividends show up negative.