Just noticed this bug using the spell "Regeneration", which is supposed to heal 1 health per turn, +1 per Life Shard.
I had cast the spell on my sovereign. In the meantime, I recruited 2 more champions and obtained a single life shard. I also cast regeneration on my two champions, both casts occurring after the shard had been constructed. In both the tactical combat and in the overland map, the 2 champions receive 2 health per turn, while the sovereign received only 1 per turn (who had Regeneration cast prior to the construction of the shard).
By dispelling then recasting the spell regeneration, the sovereign was able to receive the amount indicated in the tooltip.
This bug is not exclusive to the spell regeneration. I was able to create the same situation with "Stoneskin" and earth shards - so it appears common to any maintained spell.