This problem persists ONLY in Robinhood accounts, other portfolios seem to be unaffected (Chase shows up correctly).
So, for the Robinhood, the 'average Cost' shows an incorrect value both in the app and on the web. It appears that the 'average cost' is calculated by dividing the 'cost/share' by the number of shares which is incorrect.
Maybe Robinhood API already sends you the average cost info? If I go to the "Lot Details" to edit it manually and enter the average share price into the 'cost/share' field, I see that YahooFinance is dividing this number by the number of shares. You need to either rename that field to "Cost" and pull the total cost a user spent on that ticker, or fix your formula. 'cost per share' should be equal to the 'average cost'.

Now, Robinhood numbers are completely messed up because AvgCost is totally off. Please fix it ASAP, you had a fantastic product that I loved using, until this one bug.

PS I tried relinking - it didn't help.