My description, as entered in IMC, is M=LB|D=20181115. This represents the location and date the map was made, so I can keep track of map updates.
I did as you suggested (D=A) and the description appeared in the Simrad. This didn't make sense as the only change was less characters in the description. So I redid the map with the original characters. This also worked in the evo3.
I'm happy that it is now working but have no explanation of what caused the problem originally.
Thanks for the suggestion.