As an example of how Investopedia mishandles interest payments on cash during its end of month processing, consider my (DieWurst's) experience in the past few days. On 4/29 I made some trades that caused me to go from a small positive cash balance to a negative balance of -$631,830.55. On May 1, I found an entry in my corporate action history of an interest charge of -$136.63. This charge correctly reflects the interest I should be charged for buying stocks on margin - 29 days of a small positive balance credited at 1%, less one day of a -$631,830.55 balance charged at 8%.
The problem? Investopedia hasn't adjusted my portfolio balance at all. I still have a cash balance of -$631,830.55. This appears to be a universal phenomenon - for example, the players who joined the game just to observe and have never made any trades should be receiving $833.33 per month in interest, but all of their account balances are stuck at $1,000,000. The 1% interest that the game's rules says they should be receiving has never appeared in their accounts.
This is not a small mistake. The cumulative effect on players who employ different amounts of leverage is enormous over the course of a 36 month game. To get a feel for just how big, consider that Spudd currently has a positive cash balance of $1,520,280.14, while Ransil has a negative balance of -$417,870.93. Naturally, trade activity will cause cash balances to fluctuate, but if that kind of differential persisted for an entire month, Spudd should be getting $1,266.90 in interest added to her portfolio, whereas Ransil should have $2,785.81 in subtracted from his portfolio. Unfortunately, that is not happening because of Investopedia's mistake. Over the course of a three year game, this kind of large error can easily compound into the six figure range.
The takeaway for me is that it's foolish not to use leverage when Investopedia is offering a no cost loan to do so. I leveraged my account on Monday only to prove to myself that Investopedia wasn't charging interest on margin accounts, but now that the evidence is in, I plan on maintaining a highly leveraged portfolio for the remainder of the game.
(Note: In theory, Investopedia could still fix its mistake, for example by making an interest adjustment on the final day of the game. Maybe it will, but considering how badly riddled with bugs this simulator is, I'm not going to hold my breath waiting for a fix.)