DatetimeAdd: amount cannot deal with calculated values

I want to create a custom column, that somewhere calculates with "the last day of the previous month".
(Today is 14th Febr 2024)
I tried these expressions, and these work:

datetimeSubtract(now, 14, "day") -> 31 jan 2024, ok
day(now) -> 14

But when combined, I only get an error, and the custom column is removed:

datetimeSubtract(now, day(now), "day") -> error

logically, it seems correct.
This a bug, clearly. Any form of calculation leads to this error, even this:

datetimeSubtract(now, 12+2, "day") -> error

Is there another method to get the last day of the previous month - at any given day of the current month?