From my experience the default LTM time setting appears to use the previous 12 months instead of the "trailing 12 months".
Example: If Current Period is December FY20
LTM shows DecFY19 to NovFY20 instead of JanFY20 to DecFY20
To get around this we built the metric manually using TimePeriod(0,X)+TimePeriod(-1,X) ... +TimePeriod(-11,X)