You don't develop feelings for them by keeping them at arms length. Don't reply immediately to their messages. Take a pinch of salt before you meet them, remind yourself to not be too open.
For me, one lived 100 miles away and was keeping Covid safe, so I only saw him a few times in the summer. He made it clear from the off he didn't want a relationship, so I went into it with a clear head of "this is purely fun". No gazing in each others eyes or nuzzling noses.
As I saw him so infrequently, I picked up a second fwb. He made it clear he didn't want a girlfriend, only sex. My head was straight with that. As the weeks went on, he fell for me big time, told me he loved me and couldn't get enough of me. He was clear he didn't want to be feeling this way, but he couldn't help his feelings developing as I added far more to his life than he anticipated. Eye gazing, nibbling on my shoulder, sending me songs he sang along to on what's app. It was quite heady for me and i Iet more feelings develop for him than I should have.
I ended both when I began seeing my now boyfriend this summer. But both of them are still in touch, as they appreciate my outlook and experience in life. Even tonight, the one who loved me discussed his new fwb with me and was asking for my opinions.
It's a real weird dynamic.
OP, the key is to be guarded and to know there's a lifespan on these things.