This has not been an issue previously: I searched a destination this morning and saved the result to 'My places'. Choosing 'Navigate', one route alone displayed. I searched a local postcode, which has returned three choices always, and selected 'Navigate' directly. One result again.
I have Premium Features, and 'Alternative Routes' is shown as 'In purchased pack'.
'Alternative routes' is checked in Navigation>Advanced.
I am running version 3.1.120 on my Moto G 4G Android 'phone.
I have powered my 'phone off and on to no avail.