Sometimes rally just doesn't work
As far as I can tell that happens whenever the enemy gives no exp and any enemy that respawns infinitely will give no exp and no loot. The grubs in this cause would be one of those things. It also makes it impossible to rebuild your stacks if you are using a stacking sigil.
Did TA story the other day and one of the bosses’ infinite spiders also have the issue.
You can rally from the grubs though.