CathInfo.com

Sometimes I see social media posts of Rome-approved semi-trad groups and I feel like I can relate emotionally or internally because that’s the position I used to hold before I realized what was wrong. I worry sometimes that I may be doing something wrong or holding a sinful sympathy.

Pope Pius IX was hurt much from Napoleon, to say the least.  Napoleon said to Pope Pius IX, "you hate me".  Pope Pius IX said, I pity you!

Pity, you feel sorrow for those you are led astray .

So ... I do not consider semi-trads to be the same as being adherents to a "false religion", not formally, because they haven't been condemned by papal authority, but only based on our judgment.  It's not the same as being a member of the Orthodox Church.  There are some in the Conciliar Church who err in good faith, and that's probably true of most if not all the semi-trads, as you refer to them.  I in fact call them "trads", vs. "Trads" (capital T) for actual Traditionalists.

Now, "sympathy" for (an actual) false religion is not intrinsically mortal sin ... but if it's somehow a temptation for you that might lead to religious indifferentism, it could be mortal sin if you indulge in it knowing it to be a near occasion of sin for you, where it causes you temptations against the faith.  CHRONICALLY indulging in such emotions can weaken your faith.

You're better off "emoting" for the PEOPLE in the false religious, and in particular out of sympathy for them rooted in a desire for their conversion, at which point it's not only not a sin but can even be a virtue.

So, this problem does arise in general where people's feelings of sympathy for (those in) false religions, on a NATURAL level, leads to an erosion of SUPERNATURAL faith, and as long as we keep the two very clearly distinct in our minds, it's perfectly fine to have natural sympathies, to feel sorry for people, etc. -- but we don't do violence to Church dogma regarding salvation simply because we sympathize with them and feel sorry for them.