As suggested by Thorsten on the mailing list, switching the wing flexer property rule to be an <autopilot> instead of a <property-rule> (i.e., running at FDM rate) fixes this issue, at least for me. See https://github.com/IskenderWang/787-family/issues/21,
https://sourceforge.net/p/flightgear/ma ... /35730689/ and https://sourceforge.net/p/flightgear/ma ... /35730663/
as well as https://github.com/IskenderWang/787-family/issues/52 for a new issue (and fix) that arises with using this configuration.