Money market accounts sold by brokerage firms are a type of mutual fund and their share value can fluctuate based on the value of the underlying assets.
FDIC insurance is not unlimited. It is subject to certain dollar limits per depositor and total coverage in any one bank can be affected by type of account (e.g. single, joint ownership, trust) as account balances for insurance purposes are tallied per depositor in each bank.
It is rare, but there have been historical instances when mutual fund money market accounts have “broken the dollar.” Brokerage based MMAs are insured by the Securities Investment Corporation (SIPC) , not the federal government.
Financial firms offering MMAs make every effort they can to maintain the dollar per share valuation.