This card has two specific bugs. One is likely very specific to Lone Rider while the other is probably a broader issue with all transform cards.
First bug:
Board state:
Olivia, buffed by herself, in first creature spot.
Lone Rider, buffed by Olivia, in second creature spot.
Step 1: Olivia attacks, deals 11 damage and heals me for 11.
Step 2: Lone Rider transforms into It That Rides as One (maintains the Olivia buff of flying, naturally has lifelink, and is a 7/7 - naturally a 4/4 with Olivia’s +3/+3 buff).
Step 3: It That Rides as One attacks, deals 1 damage and heals me for 1.
This is very confusing, I could see if it didn’t register the mid combat transform but it also seems to disregard the initial buff from Olivia as well. In subsequent turns, It That Rides as One attacks and deals the 7 damage and heals me for 7.
Second Bug:
Board state:
Olivia, buffed by herself, in first creature spot.
It That Rides as One, buffed by Olivia, in second creature spot.
Step 1: Cast a Lone Rider, it gets buffed by Olivia to become a 4/4.
Step 2: Combat, Olivia attacks.
Step 3: Lone Rider transforms into It That Rides as One and reinforces the original one but only becomes an 11/11.
In this case, the second Lone Rider appears to lose it’s Olivia buff when reinforcing the original (base 4/4, Olivia’s +3/+3, reinforcement of a second base 4/4). I have cast two Lone Riders in one turn with Olivia out. First cast is a 1/1, gets +3/+3 from Olivia, reinforced for an additional 1/1, and a second Olivia buff of +3/+3 for a total of an 8/8. Once this transforms, it maintains both Olivia buffs to become a 14/14. It seems the issue is with it transforming to reinforce an already transformed creature.