First of all it is quite common that the mortar pointing to a flashing can fall out over time and this is part of house maintenance.
Likewise some weather erosion on exposed elevations, but this is only after15 to 18 years not 18 months!
Soft mortar or to use the correct term weak-mix mortar is a growing problem across all housebuilders (who is yours?) and yet the main warranty provider the NHBC is doing nothing to address it. The primary cause is cement replacement additive being used. Materials such as GGBS for example. Eve the NHBC';s own "expert" concludes this is the probable reason for the growing incidence of failing mortar in new homes over the last 8 year or so.
The issue is so bad and has serious structural implications for owners that today I set up a
'Campaign Against Weak Mix Mortar' Facebook Group to raise awareness of the issue and so people like you can share their experiences.
I am afraid you are at the start of a long journey. You have already experienced the denial and reassurance.
Write to your housebuilder and insist the mortar is tested at an independent laboratory!
When it has insufficient cement (it should be 1 cement to 5.5 sand) make a warranty claim.
You are not alone. Check out Twitter for "#weakmortar" or #weakmixmortar