Hopper minecarts call HopperBlockEntity.suckInItems even when they are full. This causes them to send comparator updates every tick around extracted inventories even if the cart is full.

By contrast, regular hoppers do not because they check whether they are full before trying to suck in items from above.

Test:

1. Build setup in the image [shulker box above hopper minecart and hopper and 2 comparator update detectors]
2. Fill both hopper and hopper minecart with full item stacks [in this case 5 stacks of 64 slime blocks]
3. Place item stack in both shulker boxes [most block inventories will work]

Expected: Neither comparator update detector gets triggered - after player interaction
Observed: Hopper cart comparator update detector gets triggered constantly.

Additional notes:

The fix should ideally be an isFull check before MinecartHopper.suckInItems() is called. This will likely also reduce lag in cases where hopper minecarts are full, most frequently being hopper minecarts in 2x hopper speed box unloaders.

The fix should not patch inventory hopper arrangements which currently do send comparator updates. These include: player interaction, normal transferring, and nonempty extracted inventory + unfull hopper/hopper minecart inventories which do not result in items being transferred. Also said arrangements should also still work with hopper minecarts after the fix.

This bug likely affects many more versions than currently listed.