fmt library is ignored

So is interpreted (and does not appear in the HTML, passed to PD4ML), but the resulting numeric data format is invalid?

Does depend on Locale settings of user session? If so, you need to propagate the session ID with https://my_server/ctx/struts_action.do?arg=val1 and other URLs.

Here is some info how to do that:
http://pd4ml.com/support/post500.html?hilit=jsessionid#p500