The feel function mapper has a
SimpleDateFormat instance as class field which it uses unsynchronized.

From the SimpleDateFormat docs:

Date formats are not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

The SimpleDateFormat for example uses a calendar field which is modified during parsing of a date.

AT:

• the FeelFunctionMapper uses the SimpleDateFormat in a thread safe way or a thread safe alternative