Reiner being "dishonest" to Porco
Reiner had been doubting about his qualification of being a warrior(and deserving his mom's love) ever since Marcel told him his secret. The failure of Paradis mission strengthened the belief that he didn't deserve to be a warrior, because Marcel had stole this from Porco.
Reiner must have felt that he at least have to protect Porco at all cost, for Marcel's sake, after Porco inherited the Jaw Titan against Marcel's wish. Reiner felt he owed Porco that. He low-key kept an eye out for Porco as a way to repay his debt to Marcel saving his life. And I believe Reiner did care about Porco, even if it's not for Marcel's sake. He would endure Porco's sarcasm and protect him from the painful secret of Marcel. Reiner was possibly afraid that the truth would taint Marcel's image in Porco's mind, and he waited for Porco to see the memory of Marcel's secret by himself in time.
This "dishonest and insincere" attitude of Reiner irritated Porco so much and he felt so frustrated. These two, even when they did care about the other, it was almost impossible for them to understand and help each other properly.
All these misunderstandings, obstables and tangling contribute to the depth of gallirei relationship. The more they have to go through, the sweeter the end can be, if there will ever be a good ending for them.