I have some NPIOs in Classic, such as Other Work, Services, Products. Associated Financial Transactions do not show under Transactions in MUX.
This is working as designed.
MUX will only show transactions posted to investments and objects available in MUX.
It is recommended that if you are using MUX, to recreate the NPIOs as Custom Investment Types in MUX so you can fully leverage all the additional functionality available in MUX.