I treat my lifetime ISA like a normal ISA - fully taxable by the IRS, just like a plain taxable brokerage account.
Bonus is "other income", taxed at income rates
Interest (if a cash LISA) is taxed as interest just like a bank account or bond, at income rates
Dividends and capital gains (if a S&S LISA) is taxed at either income rates (ordinary dividends and short term capital gains) or the lower capital gains rates (qualified dividends and long term capital gains). Your UK broker won't provide reporting on whether they're short/long term or ordinary/qualified dividends, since UK tax doesn't have those concepts. Not too hard to track in Excel yourself.
And hopefully any investments within the LISA aren't PFICs, since the LISA wrapper doesn't shelter them from the IRS's onerous reporting and punitive tax on PFICs.