I opened three lines on Tmo back on July 24rd (ported numbers over) and two lines had the 1GB LTE data plans on them (3rd was unlimited). Billing period was from July 24th to Aug 6th, 14 days, and I tracked all usage via T-Mobile.
All three lines had everything unlimited (obviously) and two lines were listed as having 1GB LTE data, with one line hitting 925MB before Aug 7th with no throttling (the 2nd line had 770MB by that time). Current use showed both as having 1GB LTE data limit/cap with no prorating to them.
Next billing period started Aug 7th with new limits given/reset. Bill came in last week with prorated charges for that 14 day period and the full bill period.
As I mentioned, I just joined Tmo from Sprint in July, so maybe they changed it up now with their Simple Choice plans or some other plan provisioning.