Seems like you gotta close the MARK page first as you said.
Upon opening the MARK page while you were on FCR but no SPI set (so TMS up hasnt been pressed, nothing designated) it returns the FCR to the AGR page.
manually selecting the GM mode again and slewing the Radar will work, yet TMS input is ignored as the Mark page is still open.
MARK_FCR_TMS_Test.trk