Why Did My Money Market Mutual Fund Investment Drop From ₦100,000 to ₦82,000 in Nigeria?

Your question is a good one because a money market mutual fund is generally expected to preserve capital, so seeing ₦100,000 become ₦82,000 is not normal under ordinary circumstances.
Here are the most likely explanations:
The investment was not actually in a Money Market Fund. Many people confuse money market funds with equity funds, balanced funds, or bond funds. Those other funds can fluctuate significantly in value.
Part of the investment was redeemed (withdrawn). If ₦18,000 was withdrawn, or fees or charges were deducted (though this would rarely amount to that much), the balance would reduce.
There was a unit price adjustment or reporting issue. Sometimes the app may temporarily display an incorrect balance or the number of units rather than the full value. This should be verified with the fund manager.
An exceptional event. Although money market funds are considered low risk, they are not guaranteed. In very rare cases, a fund can suffer losses if an issuer of a security in the portfolio defaults or there is another extraordinary market event. However, a sudden 18% loss in a Nigerian money market fund would be highly unusual.
What you should do
Confirm that the investment is indeed the Stanbic IBTC Money Market Fund and not another fund.
Check the transaction history to see whether any redemption or transfer occurred.
Contact the fund manager or your investment platform and ask for:
Your statement of account.
The number of units you own.
The current unit price.
An explanation for the reduction from ₦100,000 to ₦82,000.
If you are referring to the Stanbic IBTC Money Market Fund, a drop from ₦100,000 to ₦82,000 would be highly unusual and warrants immediate clarification from Stanbic IBTC.