It should be

DateTimeFormatter f = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSXXX");

//or

DateTimeFormatter f = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSz");

instead of

DateTimeFormatter f = DateTimeFormatter.ofPattern("yyyy-MM-ddTHH:mm:ss.SSSZ");

From JAVADoc:

Offset X and x: This formats the offset based on the number of pattern letters. One letter outputs just the hour, such as '+01', unless the minute is non-zero in which case the minute is also output, such as '+0130'. Two letters outputs the hour and minute, without a colon, such as '+0130'. Three letters outputs the hour and minute, with a colon, such as '+01:30'. Four letters outputs the hour and minute and optional second, without a colon, such as '+013015'. Five letters outputs the hour and minute and optional second, with a colon, such as '+01:30:15'. Six or more letters throws IllegalArgumentException. Pattern letter 'X' (upper case) will output 'Z' when the offset to be output would be zero, whereas pattern letter 'x' (lower case) will output '+00', '+0000', or '+00:00'.

What is happening here is that when you parse the string, an additional resolution phase happens, and turns the yyyy format specifier representing the "Year Of Era" temporal field into a "Year" temporal field in the returned TemporalAccessor, and you end up with (yearMonthTemp.toString()):

{Year=2001, MonthOfYear=2},ISO

When you format, the yyyy format specifier expects to format a "Year Of Era" temporal field, but the temporal accessor doesn't have it, as you can clearly see above.

If one needed to change the format of a Temporal without knowing the formats head of time, eg from yyyyMM to MMM-yyyy, you would need two different DateTimeFormatters. The formatters would be completely compatible in terms of the fields that they parse and the temporal objects they produce.

If all you want to do is changing formats, you can set the resolver style to STRICT (by default this is SMART):

DateTimeFormatter.ofPattern("yyyyMM")
    .withResolverStyle(ResolverStyle.STRICT);

Though I cannot find documentation for this, I have found that this will prevent it from automatically changing "Year Of Era" to "Year" (which is typically denoted uuuu). From my testing, every parsed temporal field will be present in the result. Assuming that you have the same format specifiers in the formatting DateFormatter, it will get the same temporal fields successfully.

Note that this also means that you cannot easily get something like a YearMonth from this DateTimeFormatter, because YearMonth.parse/YearMonth.from expects a "Year". not "Year Of Era".

// does not work
System.out.println(YearMonth.from(yearMonthTemp));