I have the standard Denso navigation module YIB500090 installed, I 'borrowed' the Nav. Disc from my Sons L319 to confirm disc or reader issue as the installed one was giving read errors. His disc evidently has later firmware which seemed to install successfully but since then, the moving map hasn't been as smooth and when under route guidance, the turn arrows are missing from the green 'road sign'.
I've tried with the original disc - no change.
Swapping Nav. units - the donor (YIB500070) works OK in both vehicles, with either disc [although the read errors persist when using the L322 disc in either unit, so it's probably the disc at fault, not the navigation unit], the suspect nav. unit behaves the same in either vehicle with either disc.
I think it's clear that there is a software / firmware issue. The YIA500010 update disc only deals with HDLF CAN / MOST issues, not problems with the navigation unit itself so would be unlikely to help.
Has anyone any idea of if it's possible and how to force a firmware update for the same or earlier version?
It can't be done by SDD - nor the '753' diagnostic menu, I've checked.

Just in case anyone is waiting to see if there is any resolution to this, I bought a 'spares or repairs' unit on fleabay that had a faulty DVD drive., swapped the drive from mine (after taking photos of all the stored destination pages) and everything is back to normal. As a bonus, the unit I bought had a 2018, unofficial disc in it that works perfectly. All I need to do now is key in the 55 saved destinations....