I was actually writing a bug report very similar to this issue when HLO went offline and kicked me out of the bug reporting screen. And unfortunately, the issue is complicated enough that that means I just lost a fairly long email
Short version: Sentinel, by RAW, bases itself off your armor proficiencies when you take the dedication. The HLO version appears to only base it off your class. This creates an order of operations problem (or the OP's bug).
Longer version:
Take a generic rogue. Give him sentinel at 2. It "sees" his light armor proficiency, and gives him medium armor proficiency that will track it. If he takes armor proficiency at 3, he gets heavy armor at trained, and it doesn't progress.
Now reverse the order: give him armor proficiency at 1 via the human feat general training (or move sentinel up to level 4): his light armor progresses normally, medium armor stays at trained, and the sentinel granted by heavy armor will track his light armor proficiency.
You can also duplicate this with unarmored classes such as bard, wizard, or (cloistered) cleric, by taking armor proficiency twice.